glgen: Better support for null arguments in JNI

- Modify the checks.spec file format to specify which argument is
  allowed to be null.
- Update the JniCodeEmitter to support passing null arguments for all
  pointer types as well as strings.
- Update the EGL/GLES checks.spec files and stub files to allow null
  arguments through the JNI layer where the spec allows it.

Bug 9321681

Change-Id: I7d8e7319b43da95993516aa3ac5318b971a1ff92