BroadcastQueue: max pending, misc fixes.

To avoid paused or delayed queues from getting backed up too far,
add a "MAX_PENDING_BROADCASTS" release valve.

Ignore ANR timeouts while the system is booting.

Ignore broadcast state changes once it's already reached a terminal
state to handle racing timeout messages.

Fix isEmpty() bug where caller wanted to check for empty-and-idle.

Bug: 245771249
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: If5a5898bbfc0005e1c33a31aaaf05c8eab7f90fb