BroadcastQueue: apply singleton policy at entry.

We originally intended for BroadcastSkipPolicy to make its skipping
verdicts without any side-effects, but logic that mutated manifest
receivers targeted at "singleton" processes accidentally leaked in.

This change adds tests to confirm the expected behavior for an
example singleton process, and then lifts the mutating logic into
a place where it can be applied early and consistently.

Bug: 244706927
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Test: atest FrameworksMockingServicesTests:BroadcastQueueModernImplTest
Change-Id: Iadeacabaafe5bf7267ff242f439729907e0fe32b