Stop using OMX_COLOR_FormatAndroidPrivateStart.

This removes the use (and definition) of the Android-private range of
OMX color formats from Stagefright.  Instead we will just be
interpreting the video color format of an OMX port as an Android pixel
format if the port is in native buffer mode.

Change-Id: I3ea50703336a88249e7563bc7022dfedbeac506e