mm-video-v4l2: vdec: Add support for describing color format

Android flexible YUV support provides apps with a generic
YUV format that can be derived from vendor-specific YUV
formats (linear formats only).
This requires the component to describe the vendor-specific
color-formats with the following info:
[1] Stride/slice-height
[2] Number of planes, and for each plane:
    [a] Byte-offset from start of image
    [b] y-step and x-step
    [c] Horizonal and Vertical subsampling

This change specifically describes the Venus-NV12 format.

Amended by Lajos Molnar <lajos@google.com>: removed the
error log on the normal scenario when describing
standard OMX formats. Adjusted image width & height.

Bug: 17906609
Change-Id: I812e207ecd05d67475fa0495225129a0ba346d15