Use MessageTracker for private group invitation messages

Depends on !405.

See merge request !412

Factor MessageTracker out of BdfIncomingMessageHook

The branch moves the MessageTracker implementation from BdfIncomingMessageHook to a separate class. This will allow the private group invitation client to track messages from classes other than the one that implements the delivery hook.

I've also fixed a couple of bugs, removed some redundant code from the validation manager, and added null-safety annotations (which is how I noticed the bugs).

See merge request !405

Add visibility and OPTIONS button to private group join notices

![device-2016-11-11-180658](/uploads/e00d175a1e1f34307c5f0d80fa0d1cdf/device-2016-11-11-180658.png)
![device-2016-11-11-181325](/uploads/0f6010094d529a4f151db8ebce974885/device-2016-11-11-181325.png)

Closes #732

See merge request !408

Made private group timestamp greater than that of latest message

I did not change `getPreviousMsgId()` to `getPreviousMsgHeader()` because there doesn't seem to be a need for it anymore.

Closes #755

See merge request !409

Add onboarding dialog for revealing contacts screen

This MR is based on !406.

![device-2016-11-11-162417](/uploads/8df831905eebf41ecdbe368890cdd429/device-2016-11-11-162417.png)

See merge request !407

Add UI for revealing contacts within a private group

This addresses one part of #732. Join notices and onboarding will follow in separate MRs.

Although this MR is technically not based on !402, it does require it to be merged first to compile.

![device-2016-11-11-160454](/uploads/8cb6d8bdd0f3761657875024739e730a/device-2016-11-11-160454.png)

See merge request !406

Mark relationship visible when syncing group with peer

This branch updates the private group invitation protocol to use @grote's new method for marking a contact relationship visible to the group.

I've changed the method slightly because the protocol state machine allows us to leave and re-enter the BOTH_JOINED state (see diagram on #659), so the relationship may already be visible when the method is called. In that case the visibility isn't updated, so we stick with whichever of revealed-by-us and revealed-by-contact happened first.

See merge request !402

Prepare UI for revealing contacts

This changes the visibility of some methods (that need to be accessed from another package), removes unnecessary abstractions and fixes erroneous static import of GroupId constant.

See merge request !404

This changes the visibility of some methods, removes unnecessary
abstractions and fixes static import of GroupId constant.

Add new Exception handler

While working on #732 I again needed to use a `UiResultExceptionHandler`  when I actually don't need to return a result. We have some other places in the code like this. So I introduced a `UiExceptionHandler` without the result part and used it where appropriate. While I was touching some classes, I also added the new annotations.

This MR includes another small commit that notifies only on local group messages. Joining a group is a message as well and without this change, you are notified about a new message when you yourself joined your newly created group.

See merge request !403

Also add NotNull annotation to classes that were touched

Add a controller for contact selection lists

See merge request !401

Add support for revealing contacts to the PrivateGroupManager

This also adds three integration tests and improves some small details here and there in the private group client.

Prerequisite for #732.

See merge request !396

Preserve the order of descriptors in QR code payloads

This fixes a regression caused by my recent changes to the Payload class.

Closes #748

See merge request !399

Don't crash if camera is reopened or surface is recreated

This branch fixes the crash is described in #720, which can be reproduced easily by scanning a QR code and failing to connect (for example, scan a screenshot of a QR code from a device that's no longer listening). When the camera view becomes visible again after trying to connect, its surfaceCreated() callback is called again with the same surface. An IllegalStateException added in !340 causes the crash.

Closes #720

See merge request !397

This also adds two integration tests and improves some small details

Make the feed pager respond to events, block notifications

This branch fixes a bug I found while working on #705: FeedPostPagerFragment doesn't start or stop the controller, so it doesn't load newly received posts or block notifications, unlike FeedFragment.

See merge request !398

Encode transport properties more compactly in QR codes

The [original BQP spec](https://code.briarproject.org/akwizgran/briar/wikis/BQP) described a compact encoding for transport properties, with the goal of making the QR code as small as possible. At some point during the implementation, I asked @str4d to use TransportIds and TransportProperties instead, as described in the [current spec](https://code.briarproject.org/akwizgran/briar-spec/blob/master/protocols/BQP.md). That was a mistake.

Using the original format reduces the payload from 60 to 34 bytes (43% smaller) for Bluetooth only, and from 96 to 49 bytes (49% smaller) for Bluetooth and LAN. This makes it easier to scan codes from low-resolution screens using fixed-focus and/or low-resolution cameras. Using this branch I can exchange codes between the Sony Xperia Tipo (320x480 screen, fixed focus, 640x480 preview size) and the Huawei Ascend Y300 (480x800 screen, infinity focus, 1280x720 preview size).

This also removes an obstacle to implementing #558, as TransportIds are no longer included in QR codes.

Closes #346.

See merge request !394