BackupManagerService: Unbreak "adb restore"

The new restoreOneFile() logic aborts the loop whenever the result
isn't an actual successful restore. The problem is that a normal
backup will have plenty of non-restore cycles (APKs that are already
present being the most common), so this breaks regular "adb restore".

Go back to the original logic of completing the loop as long as the tar
file has data (or an explicit abort is generated)

Change-Id: I808cdc1c3b374e29d8438c7051e37c24a7a9d991