lavc/mediacodecdec: switch to new decoding API

Using the new API gives the decoder the ability to produce
N frames per input packet. This is particularly useful with
mpeg2 decoders on some android devices, which automatically
deinterlace video and produce one frame per field.

Signed-off-by: Aman Gupta <aman@tmm1.net>
Signed-off-by: Matthieu Bouron <matthieu.bouron@gmail.com>