Of course, not all devices support Bluetooth...

This follows the principle of least surprise: the default identity
doesn't change when a new identity is created.

Hopefully this gives users a better understanding of whether their
messages and contacts are accessible than "Briar is running" and "Quit".

This give a simple and usable interface; we can worry about collapsing
blocks of read messages later.

GroupActivity needs to receive a similar treatment.

This avoids a possible problem where the app expires while it's running
and every activity shows the expiry warning, leaving the user with no
way to quit.

See
https://developer.android.com/guide/topics/ui/notifiers/notifications.html#NotificationResponse

This will allow any activity to show the expiry warning and password
prompt, and to start and stop BriarService.

SplashScreenActivity, SetupActivity, ExpiryActivity and PasswordActivity
are special cases.