[2/3] Factor out current BroadcastQueue implementation.

Upcoming work will look at making significant changes to the
BroadcastQueue internals, so this change factors out the current
implementation to aid A/B testing.

This surgical change is designed to be a no-op refactoring with no
actual behavior changes.

Bug: 243656033
Test: atest CtsContentTestCases:BroadcastReceiverTest
Change-Id: Ic7aaf30f4c7cc062e502cfb074c40939c6792a4f