LibAAH_RTP: Change names to prepare for refactor.

Rename AAH_TXSender to AAH_TXGroup in preparation for refactoring to
support unicast retransmission.

Signed-off-by: John Grossman <johngro@google.com>
Change-Id: I3984db27d1c61c6155d5d7cb9c38eead421b9249