VT: Add auto downgrade function

Some network requires force-downgrading to audio call when video packet status is bad.

1. ARTPSource reports bitrate value for every unit time.
   If low bitrate duration stays long, ARTPConnection triggers auto downgrading message.
   The way of passing message is same as TSFB & PSFB.

2. Add auto downgrade callback message when connection lost.

Merged-in: I04e3f4e1955852c61540f35c763a40be3e7b639b
Change-Id: I04e3f4e1955852c61540f35c763a40be3e7b639b
Signed-off-by: Kim Sungyeon <sy85.kim@samsung.com>