Move gradle witness dependency verification into dedicated files

See merge request !892

This also adds a script for makes upgrading dependency checksums so much easier

Replace CheckBoxPreference with the more standard SwitchPreference

See merge request !891

# Conflicts:
#   briar-android/src/main/java/org/briarproject/briar/android/settings/SettingsFragment.java

Screen Lock Feature

See merge request !870

Also automatically unlock if device screenlock was removed.

Remove unused constants

See merge request !889

Make back-facing camera optional in manifest

See merge request !888

This solution even works when "don't keep activities" is active

When the user removes the screen lock, the app does not get really
locked. There is no way about getting notified about this. Before users
lock the app without it getting actually locked, we rather remove the
button that was collapsed and not easy to find anyway.

This only shows the settings if a screen lock is available.
If the setting is activated, it shows a drawer item for locking.
Once locked, the screen lock secret needs to be entered to enter the
app again.

First part of #1247

Factor out Android-specific code from TorPlugin

Closes #832

See merge request briar/briar!876

Show names of countries where Tor is blocked

See merge request !886

Rename main flavor due to internal conflicts

Closes #1360

See merge request !885

Replace assert statements with AssertionErrors

See merge request briar/briar!877

Dark Theme Changes

See merge request briar/briar!884

Use full class names for loggers

See merge request briar/briar!881