Allow non-mainline package rollbacks

If3aaf8f2c454f45b9d79c9dae5a21d356e4cf029 prevented rolling back
packages that were not part of a mainline update. This made it
difficult to test and the auto PackageWatchdog test was ignored in the
meantime. We are now allowing non-mainling package rollbacks and
re-enabling the test.

I8dd7e6d1e144251830108c58f4a752c411d7295b introduced a refactoring bug
when rolling back packages, the module metadata provider was rolled
back instead. This was not caught because the test was ignored.

Test: atest RollbackTest
Bug: 120598832

Change-Id: I8fcc9492a05f0b3788ba4762ad3e3e5ac086d43b