Add some global metadata to the restore set

In addition to the signatures of each participating application, we now also
store the versionCode of each backed-up package, plus the OS version running on
the device that contributed the backup set.  We also refuse to process a backup
from a later OS revision to an earlier one, or from a later app version to an
earlier.

LocalTransport has been modified as well to be more resilient to changes in the
system's use of metadata pseudopackages.