base

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