Thanks to the team that audited CryptoCat for this idea. :-)

Very important stuff.

Fixes bug #42.

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

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.

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.