Fixes bug #7. The service is restarted after a crash even if
onStartCommand() returns START_NOT_STICKY, but after restarting the DB
key is not restored and so the app can't usefully proceed.

If the app restarts after crashing and tries to return to the activity
where it crashed, DatabaseConfig's encryption key will be null and no DB
operations can be performed. It's better to crash again in that case
than for all DB operations to fail.

Fixing bug #3 will make this irrelevant.

Also fixed a bug in addGroup(): SELECT COUNT (NULL) doesn't work.

Offered and requested message IDs are stored in the database rather than
being owned by DuplexConnections. This paves the way for moving to a
simplex transport layer.

The distinction between inbox groups and other groups can be maintained
internally, there's no need to represent it on the wire.

This can happen on Android if the app is automatically restarted after a
crash.

This allows us to avoid instantiating the plugin on unsupported
architectures.

Unit tests would've caught this.

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.

This produces a saner user experience when devices have differing
clocks.

Older devices such as the Huawei U8210 don't completely support the Java
1.6 API. Calling an unsupported method crashes the app.

Older devices such as the Huawei U8210 don't completely support the Java
1.6 API. Calling an unsupported method crashes the app.