Fix the Backup Manager's uninstall tracking

The bug was that when an app was uninstalled, the Backup Manager was
discarding its bookkeeping about that app being represented in the
device's current live backup dataset.  This in turn meant that if the
app was subsequently reinstalled, its data would not be restored from
that most-recent dataset: it would be restored from the *ancestral*
dataset if possible, or not at all.

Now the "ever backed up" state is retained correctly, and the app
will get its most-recent-data restored as expected.

Bug 7394519

Change-Id: I733cf41737765676e0a3a05fb1bcd32b165cb4ba