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 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.

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.

The new format is simpler but less efficient for small integers, short
strings and short byte arrays.

If the schema of the database is incompatible with the schema expected
by the code, the database throws a DbSchemaException. LifecycleManager
indicates the error to BriarService, which uses HomeScreenActivity to
show a notification and quit the app.