Add tracing for broadcast sending and processing.

This change instruments BroadcastQueue using asyncTrace to give us
visibility into broadcasts from a trace. There's one traced section for
the time the broadcasts spend in the queue, and another one for the time
it takes to dispatch the broadcast.

My previous solution for this bug was incomplete, so this change also
cleans up that solution (which instrumented a callsite). Last, there's a
couple minor changes to logs (fixing a typo and making a log clearer).

Bug: 33645477
Test: https://screenshot.googleplex.com/2F6SWiCtEUD

Change-Id: I27e38cd70fee144986405bae302eb04045dff06d