PixelCopy fixes

Bug: 27708453

Fixes some issues with camera sources. Previously
it was using GL_TEXTURE_2D target which doesn't
work properly if the source is YUV. It is critical
to ensure GL_TEXTURE_EXTERNAL_OES is used throughout
so the right sampler is used.

Change-Id: I0dcd8941ba08331f24809467b0e828663a38e93b