BroadcastQueue: clean up cold, empty queues.

When we notice that a process has been killed, and they have no
remaining pending broadcasts, clean up the BroadcastProcessQueue
instance that we had allocated for them, to reduce memory pressure.

Tests to verify.

Bug: 245771249
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: Ia0aadbe1d684ea2f5babff0174753aee8d27b55a