libaah_rtp: address http://b/issue?id=5755419

Introduce a heartbeat timeout for TX senders.  If none of the TX
sender's clients send any packets for this timeout period (10 minutes
right now), then the sender will stop sending keep-alive heartbeats.
This should cause RX clients to release any held media resources,
including any timed audio track.  Releasing timeed audio tracks allows
the audio system to go into its idle state, which allows the power amp
in Tungsten to power all of the way down, saving both heat and power.

Change-Id: Ib7a96d334e1064ddea3f07a6b21e6efedefc549a
Signed-off-by: John Grossman <johngro@google.com>