Prevent concurrent backup operations

We've seen cases (bug 5417779) where the transport kicked off an immediate
backup operation but then was perfectly content to allow the periodic
timer to start *another* pass concurrently while the first was still in
progress.  This wound up with the backup manager getting mightily
confused and leaking wakelock acquisitions, which is Very Bad(tm).

This patch adds a little bookkeeping so that the backup manager is aware
of backups in flight, and refuses to kick off a new one until the ongoing
one has finished.

Change-Id: If12b54f4db3effc8af36d31c58d8f9b415ddc01e