Remove TLS_SLOT_OPENGL usage on Android, use C++11 thread_local instead

It looks like we can't use bionic OPENGL TLS SLOT anymore if ANGLE is the
_system_ GLES driver. Yikes. Using C++11 thread_local seems to
work better.

Change-Id: I08cbc35deabd553d023683f8231b192556bec646
2 files changed