[pm] package removal/deletion helper

Due to entangled dependencies between package deletion and stub system
installation, I had to slipt package delete helper into two classes.

DeletePackageHelper depends on RemovePackageHelper to clear internal
data structures. It also depends on InitAndSystemPackageHelper to
re-enable stub system packages after system package deletion.

InitAndSystemPackageHelper also depends on RemovePackageHelper, to
remove the data structures created for the stub package after the
decompressed package is installed.

I've created a TODO to clean up the stub package situation.

BUG: 200616235
Test: manual
Change-Id: I3d3bce937e670ff7c13b00e0f186211a1ffb3632