Disallow parallel backups

If a backup operation is in progress
when another backup operation is requested,
the new one is skipped. We have to skip the
second one because the current BackupTransport
interface cannot support parallel backups.

Bug: 29010684
Test: Tested manually. Work on GTS test in progress.

Change-Id: I6d8f49897bb42781bafed584a4b75b3b971f75eb
1 file changed