Because the app launches in a restricted mode when restoring a backup,
dependency injection with Dagger Hilt doesn't work inside BackupAgent. This
would cause backup restore operations to fail. This issue got introduced by the
recent switch to Dagger Hilt, which has not been included in a release of Aegis
yet.
This adds an option for participating in Android's backup system. Two items are
backed up: the ``files/aegis.json`` file and the entire ``shared_prefs``
directory. The option is disabled by default and can only be enabled if
encryption is enabled as well.
I tested this with Local Transport and Seedvault. To test with Local Transport,
see: https://developer.android.com/guide/topics/data/testingbackup.
