Adding Metadata mode to SurfaceMediaSource

SurfaceMediaSource operates in metadata mode only, i.e. just the
metadata is stored in videobuffers. SurfaceMediaSource passes the Gralloc
buffer handle along with a 4 byte 'type' (indicating that the metadata
is of type GrallocSource) to the encoder as opposed to the
GrallocBuffer itself.

Related to bug id: 4529323

Change-Id: I83aebc0dd10f317658cdf70be5802dfc35a1e72d