Codec2Buffer: Initialize mediaImage to zero in GraphicView2MediaImageConverter

MediaImage2 structure was initialized only for valid planes, but entire
structure was compared to signal change in output format.
This was resulting in multiple change in output formats signalled in
dequeueOutputBuffer()

Bug: 124799741
Test: Manually verified that only one ouput format changed is signalled\
 per clip using AMediaCodec based decoder application

Change-Id: Ib4139fc80f3493e3cb2b019d3890c50b05076469