Starhub RTSP apparently does not establish time on all tracks

i.e. the "SR" RTCP packet is sent for only one of the two tracks.

fake timestamps if that's the case, previously we'd only fake timestamps
if we didn't receive _any_ "SR" packets.

Change-Id: Id63d4940d453ba6c04c62e02ab9a0ad843936bc1
related-to-bug: 5669027