Use scheduled job rather than periodic alarms for key/value backups

Instead of a runs-forever periodic alarm that drives key/value backup
passes, we instead schedule-on-demand a trigger job that will kick off
the pass after a batching interval.  The key semantic change is that
we now never wake for key/value backup work unless we've been explicitly
asked to do so.  We also use a rather longer batching interval than
was previously the case.

Bug 19536032

Change-Id: Ie377562b2812c9aeda0ee73770dfa94af6017778