Introduce <application> attribute android:fullBackupOnly={boolean}

It is quite possible for an application to be happy with having the OS
save full-data archives of its data, but still need to selectively
filter the set of saved files or otherwise participate in full-data
backup and restore.  In general we assume that any app which provides
a backup agent implementation will be directly participating via the
incremental key/value backup API; this new attribute allows an app
to tell the OS "perform full-data backup/restore for me even though
I am supplying my own agent implementation to participate."

Change-Id: I810c50d44aa683b1f23604b7d1f3e96a1722103a