Frameworks/base: Wall Werror in core/jni

Turn on -Wall -Werror in core/jni. Fix warnings.

Clang TODO: For GCC we need to turn off Wunused-but-set-variable in
the GL bindings. However, Clang doesn't have that warning and thus
complains about an unknown pragma. It is necessary to make the
pragma #ifdef-ed on the compiler being GCC.

Change-Id: I14cab48d45c2771eef0432082356c47ed44a3d7f