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.

We check for compatibility by specifying a schema version and minimum
schema version in the code, storing them in the DB, and checking whether
the DB is too old for the code or vice versa.

This may help with bug #27, but it doesn't solve it in all cases (eg the
contact's device restarting).

Dev task #52. Known but unverified identities are also supported, but
currently unused. These will be used in future for contacts who've been
introduced but not verified face to face.

This should've been changed when restricted groups were removed.

This partially fixes dev task #40. CreateIdentityActivity is also called
from AddContactActivity, but that's likely to change soon so I'm not
going to fix it now.

This way the tests won't show up as having run if they didn't.

Direct P2P connections between mobile devices are unlikely to work, and
they'd expose the social graph to surveillance.

Leaving the expiry date at 22 January until the fix is confirmed.