VT: Dropping P-frames till I-frame provided

[Problem] Video is not shown even a video call connected.
[Cause] Remote side sent I-frame few times at the beginning of the
  stream then just sending P-frames only.
  In above behavior of remote device, DUT can not show video till
  end of the call if I-frames are missed.
  Because remote device will not send next I-frame until RTCP:FIR received.
[Solution] Discards P-frames Then request a new I-frame by sending RTCP:FIR.

Bug: 173163174
Bug: 173474333

Change-Id: If81dd3c154071953a7fd0d1cd354c5432bf4cb43
Signed-off-by: Byeongjo Park <bjo.park@samsung.com>
Signed-off-by: Kim Sungyeon <sy85.kim@samsung.com>