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.
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.
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.
