Adds VPX encoding support for stagefright.

Adds VPX encoding support for stagefright.

Only following encoder settings are available
    - target bitrate
    - rate control (constant / variable)
    - frame rate
    - token partitioning
    - error resilience
    - reconstruction & loop filters

Only following color formats are recognized
    - YUV420Planar
    - YUV420SemiPlanar
    - AndroidOpaque

Following settings are not configurable by the client
    - encoding deadline is realtime
    - the algorithm interface for encoder is vp8
    - fractional bits of frame rate is discarded
    - timebase is fixed to 1/1000000

Requires libvpx to be built with encoder support enabled.
Requires openmax 1.1.2 extension headers.

Relevant tests exist in cts repo.

Change-Id: If759edb8db36acbd24dcb53d159a54e942766020
Signed-off-by: Kunter Gultekin <kuntergultekin@google.com>