BroadcastQueue: ignore dead registered receivers.

When a process dies (or is restarted), any registered receivers that
came from the old PID are no longer valid, and should be skipped.

Tests to verify, along with slight update to "default" implementation
to skip sending to a known-dead process.

Bug: 248605002
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: I9c82595fe2c729cb50213377e1808ffd4bebaa75