emulator/opengl: Remove android::Mutex.

This patch removes the dependency on android::Mutex from
<cutils/threads.h> by providing a custom implementation, which
is a simple wrapper around pthread_mutex_t / CriticalSection,
under shared/emugl/common/mutex.h

+ Provide unit tests.

Change-Id: I379ef0c480c478ab9ba5f2faaf8274267eff37ba
30 files changed