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.
