BroadcastQueue: implement per-process tracing.

To help investigate and debug our new per-process queues, this change
adds "track" based trace events, with both a general "BroadcastQueue"
track and a separate track for each runnable slot.

Note that to view any "tracks" you'll need to enable the "canary"
version of the Perfetto UI.

Bug: 245771249
Test: atest FrameworksMockingServicesTests:BroadcastQueueTest
Change-Id: I80c3b0aadf30250999738ebbfc83a7028a0d68e9