Bug #47.

Of course, not all devices support Bluetooth...

This allows the oldest LocalAuthor to be used as the default.

All JVMs must support UTF-8 encoding.

This adds complexity but will save a lot of bandwidth, as most of the
strings and byte arrays we want to send are less than 128 bytes.

The extra complexity isn't exposed outside of the serial component.

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.

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.

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

Note that this is only the generator part of Fortuna, not the
accumulator. The generator requires a seed, which is provided by a
platform-specific implementation of SeedProvider. On Linux the
implementation reads the seed from /dev/urandom.

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.