Ensure system internals hear broadcasts first.

Since the modern broadcast queue can dispatch to multiple apps in
parallel, we could end up telling third-party apps about events like
PACKAGE_ADDED before system internals have handled it, causing
developer confusion when calling into an inconsistent service.

This change ensures that all system internals handle PACKAGE_
and USER_ style broadcasts first before apps; if any internals want
to defer until after third-party apps, they can choose to register
themselves with SYSTEM_LOW_PRIORITY.

Bug: 269168765
Test: TH
Change-Id: I567d7dbb542d39de6cc6268fd568a9bd7046c34e