Initial checkin for software AVC encoder

- Since the software encoder assumes the input is YUV420 planar,
  color conversion needs to be added when the input color format
  does not meet the requirement. With this patch, I only added
  a single color conversion from YUV420 semi planar to YUV420
  planar. We can add more as we go.

Change-Id: If8640c9e5a4f73d385ae9bb2022e57f7f62b91b9