Add video extension OMX_VIDEO_CodingVP9 to support VP9 decoder.

Change OMX_Video_CodingVPX to OMX_VIDEO_CodingVP8 in order
to be consistant with official OMX_VideoExt.h and avoid the
confusion with new extension OMX_VIDEO_CodingVP9.

This change needs to be merged with changes in frameworks/av to
avoid build break due to header change.

Change-Id: Iaa2dbafa76dc24ad3cf52a9bf3dee312e3c64bfa