Fix logic error in downgrade of system apps

A logic error would allow not deleting data when uninstalling a system
app upgrade that's of a newer release than the on on the system
partition. If the system app had a database upgrade and this happened,
you might be in trouble.

Luckily it appears no one ever does this in code. You'd have to manually
get there doing:

adb uninstall -k updated.system.package

Change-Id: I3110fedf2d147975a0635c71898d985dd642a2c1