Rationalize GLES2/3 .in files.

We weren't using gl2_api.in, since gl3_api.in was a superset. And
gl3ext_api.in was empty, and Khronos has decided to keep it that way
(any GLES2/3/... extensions will be in gl2ext.h).

Since libGLES2.so will have all future backwards-compatible GLES APIs,
it makes sense to just use gl2_api.in and gl2ext_api.in to build its
contents. This changes renames gl3_api.in to gl2_api.in and updates
uses of it, and removes gl3ext_api.in and references.

Bug: 15028495
Change-Id: I0745eda27ec91ee625f03e957e4fdb22035dd781