Ensure that the TRTP retry buffer has contiguous sequence numbers

Previously, sequence numbers for audio packets were assigned by the
TX player before packets were queued to the sender.  This caused a
race between assignment of sequence numbers on audio packets and
sequence numbers on heartbeat packets.  A heartbeat could get queued
and added to the retry buffer before an audio packet with an earlier
sequence number got queued.

This CL centralizes packet sequence number assignment and insertion
into the retry buffer inside AAH_TXSender::doSendPacket_l.  It also
makes explicit what operations can be done on a TRTPPacket before
and after packing.

Change-Id: I6d02eae81061983e4def4f1b3dd7c1625467b151