Add thread safety analysis annotations.

Enable thread safety analysis annotations for clang.
See https://clang.llvm.org/docs/ThreadSafetyAnalysis.html
for instructions on using these in the source code.

Bug: 28094863
Test: annotated
frameworks/native/services/inputflinger/InputDispatcher.cpp
and enabled '-Werror' and '-Wthread-safety' clang
compiler flags in Android.bp for inputflinger.
Observed compiler errors when accessing
instance attributes without holding a lock. Also added
a compile test Mutex_test.cpp, which can be build using
m libutils_tests and run using
/data/nativetest64/libutils_tests/libutils_tests

Change-Id: I24ce111241cc339901bc45dda8b446df5299af4a