media: fix timestamp woes

Stream time base is for AVPacket, while codec time base is for AVFrame.
Good explanation found here: https://gist.github.com/yohhoy/50ea5fe868a2b3695e19

Sets decoder framerate during stream setup.
Scales decoder timestamps from container time base to codec time base.
Substracts stream start time from timestamp in decoder.
Fixes MediaStream to use AVCodecContext instead of AVStream.
Encoders gets next timestamp using AVCodecContext instead of AVStream.

Change-Id: I02620d825e3ff9488a4d1eb0f6eedc8dbd030df4