Prefer ES 2.0 with external image support vs ES 3.0 without that support

We currently attempt to use ES 3.0 shaders even if it lacks the
extension for external images.  This CL reverts back to ES 2.0 in
the event that we find that the OES_EGL_image_external_essl3 is not
present.

Test: no change in graphics output
Bug: 111220579
Change-Id: I6bd7145026d9686cfcf49393a19580208db8a367