Implement fast GetCurrentThreadId() function. This function is 7-8x faster than GetCurrentThreadUniqueId() on Android. This CL is required for future CLs related with Global and Context mutexes. Test: angle_unittests --gtest_filter=SystemUtils.GetCurrentThreadId Bug: angleproject:8101 Change-Id: I4023b83510708ce242f4863b5cb12c6eef15e0a1 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4401939 Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Igor Nazarov <i.nazarov@samsung.com>