Added VideoSourceDownSampler

VideoSourceDownSampler takes in a real video source. The MediaSource
interface is then implemented by shortcircuiting calls to this real source,
except for the read() function. For read we first call read of the real
source, then downsample the frame to the desired size and return that.

Change-Id: I6e1b377e043e136a70a7b03792eb9165bbe26398