Fixes to full-backup scheduling edge cases

If a scheduled full-data backup was attempted but the device had not yet
run the primary key/value backup pass, the full-data backup scheduler
would wind up in a bad state and potentially never retry until reboot.

We now properly reschedule a future retry, using the key/value
scheduling batch quantum as a backoff to be sure to give it a chance
to run before the next time full data is attempted.

Change-Id: Ic7eb7a7940fe6380f40d04813a46fc336e95815e