To help assess latency, dump the time a broadcast was enqueued.

Previously we only dumped the time when a broadcast started dispatch.
Start tracking the time when a broadcast was enqueued so that
we can determine whether the broadcast queue was stalled and for
how long.  Together with other information in the dump, this can help
to ascertain whether broadcast latency is being caused by the sender,
the dispatcher, or the receiver.

Bug: 17695520
Change-Id: I4f2e4bb92f9178f5d095b9124a15eb2e5f13b9f6