Fix uninstallation tracking in the Backup Manager

This never worked properly; now it does.  We also no longer
do a redundant pair of remove/add operations when a package is
updated.

Bonus memory savings: we were keeping sets of ApplicationInfo
objects as part of the ongoing bookkeeping, but those were no longer
being used for anything other than the package names.  That's been
tossed out now and only the name strings are now used; hooray for
memory savings!

Change-Id: I4c6e592a1680e28550bcb4f76789260ded22280d