This also adds unit tests to prevent regressions like this in the
future.

Try harder to find suitable camera parameters

This branch fixes QR code scanning on the Galaxy Nexus running Cyanogen Mod 12.1 (Android 5.1.1), without breaking QR code scanning on any of the other test devices.

The problem on the Galaxy Nexus was that the selected scene mode was overriding the selected focus mode, so we asked for continuous picture mode but got macro mode. Macro mode requires startAutoFocus() to be called, but we weren't calling it because we'd asked for continuous picture mode.

The fix for that problem is to query the focus mode after applying the parameters and call startAutoFocus() based on the actual mode rather than the requested mode.

But then I discovered another problem: barcode scene mode was setting the flash to auto, so in low light the flash was turning on and off while trying to scan QR codes. That might work well for printed QR codes, but it's terrible when scanning from a screen.

The fix for the new problem is to select barcode scene mode, then try to disable the flash, and if that fails, reset the scene mode. Then we pick the best available video stabilisation, focus mode and preview size.

On the Galaxy Nexus with CM 12.1, that means we use continuous picture mode instead of barcode scene mode, which works fine. All the other test devices pick the same settings as before.

See merge request !321

Check whether File#listFiles() returns null

The docs say this can happen if there's an I/O error. Also fixed a throw-in-finally-block warning and renamed an IoUtils method to make its contract clearer.

See merge request !328

The docs say this can happen for a directory if there's an I/O error.

Remove base32 TODOs, fix a potential NPE

I thought we'd be able to get higher data density in QR codes by using base32 instead of base64, allowing the QR code to use alphanumeric mode instead of byte mode. But I tried it, and although the QR code does use alphanumeric mode, it comes out at exactly the same size (see the 346-use-base32-for-qr-codes branch). So this MR removes the TODOs and fixes a potential NPE I spotted while working on the other branch.

See merge request !327

Recreate cache directory after deleting app data

This fixes a warning on the Moto G:
```
E/libEGL: error creating cache file /data/data/org.briarproject/cache/com.android.opengl.shaders_cache: No such file or directory (2)
```

@grote might possibly be related to hardware rendering issues.

See merge request !326

Add more introduction tests for fake MAC and modified timestamp

Closes #627, #669

See merge request !315

Add first onboarding screen

When the user enters a private conversation after adding her second
contact, an onboarding screen will be shown highlighting the possibility
of introducing the contacts to each other.

![intro2](/uploads/b2cd4377f39974ca1fbd95649558e487/intro2.gif)

Closes #357

See merge request !324

When the user enters a private conversation after adding her second
contact, an onboarding screen will be shown highlighting the possibility
of introducing the contacts to each other.

Emoji

This MR introduces a custom Emoji implementation to Briar for devices that do not support Emoji sufficiently. It is heavily based on Signal's implementation. Hence, the license for the Android part has been changed to GPLv3.

So far, emoji input is only supported for forums and private conversations that both rely on the same `TextInputView`.

![emoji2](/uploads/f69af031e72efb0bd6f0f67a9574d11f/emoji2.gif)

See merge request !317

Add functionality to save and restore recently used Emojis

Update emoji and add new categories based on AOSP's XML file

Don't offer messages back to the sender

Closes #538

See merge request !323

Avoid repeated author status lookups

Closes #625

See merge request !322

Include stream number in stream header nonce

See the corresponding change in the BTP spec for an explanation:
https://code.briarproject.org/akwizgran/briar-spec/commit/388e1d23c04c2f6954d13d36ffc231611a3c6fa9

Closes #329

See merge request !320

Fix potential NPE in AndroidExecutorImpl

Always wait for the startup latch to ensure the background thread's Handler is initialised before using it.

Closes #665.

See merge request !319

Don't throw an exception if the client has no delivery hook

Closes #664.

See merge request !318

Clean HTML from RSS feeds with Jsoup and Show Link Warning

This adds HTML cleaning with the Jsoup library based on a whitelist. The resulting HTML is then used to create a `Spannable` in Android. This spannable is traversed and URLs are replaced by a custom dialog fragment that shows the following warning.

![device-2016-09-07-154936](/uploads/87a9800bde2676b0f2ddf5a894aa04eb/device-2016-09-07-154936.png)

Closes #569

See merge request !311

Refactor ValidationManager and fix some bugs

The validation manager uses a pattern of throwing an exception from within a transaction, then trying to ensure that the transaction gets committed anyway by setting it complete in a finally block. This is a bad idea - if the intentional exception isn't thrown but a later method throws an unintentional exception, the changes up to that point will be committed instead of rolled back.

I reworked the control flow to remove this pattern, and in the process realised that the validation manager was doing a lot of work to provide objects to attach to MessageStateChangedEvents, which are only used by tests these days. So I fixed that and a couple of other issues.

The VALID state is no longer needed and has been removed.

Closes #619

See merge request !313

Introduce a DestroyableActivity for UI result handlers



See merge request !314

Resolve Outstanding Blog Issues

This MR has several commits which fix individual small issues. The biggest commit is a refactoring of the `BlogActivity`, so its `ViewPager` can be used also for the Blog feed.

Closes #637

See merge request !309

Merge branch '610-result-handlers-can-return-results-to-destroyed-activities-fragments' into '637-effect-of-touching-a-post-is-inconsistent'

Check if activity is still alive before returning results to it

Closes #610

See merge request !312