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.

Private messages are now the same as group messages, but groups can be
private or public. When a contact is added, a private group is created
and designated as the inbox for exchanging private messages with the
contact.

Fixes issue #3611920, but keep the issue open because more activities
will be added.

Android doesn't currently store bundles persistently, so it's premature
to protect against accidental information leaks through persistent
bundle storage. Protecting against deliberate information leaks by the
OS is probably futile, so there's currently no need for bundle
encryption.