screenrecord: Add raw frames output format

This works like the frames output format but without a header so that
the output can be piped directly to mplayer:

adb shell screenrecord --output-format=raw-frames - | mplayer -demuxer
rawvideo -rawvideo w=320:h=240:format=rgb24 -

Change-Id: Ib5ed508fdca3a5d63e973c5189b3677c2d92b43f