Don't forget to clear "backup in progress" state when finishing a pass

If a backup pass had been skipped (either because the transport was unavailable
or -- in a common case! -- because there was simply no work pending when the
periodic backup check fired), we were forgetting to reset the "backup currently
in progress" flag.  Once we'd done that, the device would *NEVER* perform a
backup until it was rebooted, since it would forever think that there was one
currently in operation that must not be interfered with.

Change-Id: I0d6d7375dc6de99b599222a449934e70fe13ebb9