Add app version to the backup metadata

We now record the version number of the app (drawn from its manifest versionCode
attribute) along with its signatures.  At restore time, we compare the version
associated with the restore set with the version present on the device.  If the
restore set is from a newer version of the app than is present on device, we do
not perform the restore operation.

Also fix the pending-backup iteration in 'dumpsys backup'.