Frameworks/native: Add pragmas to glgen headers

The code currently being generated by glgen is not the cleanest, but
we would like to get core/jni onto -Werror. Thus add pragmas turning
off warnings locally to the C headers.

Also fix signed-comparison in two functions.

Longer term TODO: Emit clean code.

Change-Id: Iee8582f8c0c1de076d64851d3b6ca467afd5bc43