LibAAH_RTP: Get rid of PipeEvent

Bionic/Android support eventfd, so there is really no reason to have
PipeEvent around any more.  This change gets rid of it in LibAAH_RTP
and replaces it with eventfds.

Change-Id: I841fcb71bf5015d521d7517c69f44eac0ea92278
Signed-off-by: John Grossman <johngro@google.com>