bridge_native_rtp: Keep rtp instance refs on bridge_channel

There have been reports of deadlocks caused by an attempt to send a frame
to a channel's rtp instance after the channel has left the native bridge
and been destroyed.  This patch effectively causes the bridge channel to
keep a reference to the glue and both the audio and video rtp instances
so what gets started will get stopped.

ASTERISK-26978 #close
Reported-by: Ross Beer

Change-Id: I9e1ac49fa4af68d64826ccccd152593cf8cdb21a