Allow updated system apps to retain priority

Previously any updated system apps would not be able to have a greater
than 0 priority on an activity intent filter. Moving the priority check
later in the package scanning allows it to apply to updated system
packages as well.

Cherry-pick from gingerbread branch

Bug: 2572398
Change-Id: I95d8b6360bf7a3f39cd7a1ff09e1ee57e11583d8