res_rtp_asterisk: Queue video update on picture loss indication.

The previous payload specific feedback handling was very single
minded in that it just assumed everything should trigger a video
update. This was changed but the handling of picture loss indication
was not added. The result was that video may not flow. This change
adds it explicitly in.

Change-Id: I1894be02e39ee10a0af841b5a1dca5f0ec7d60b6