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
diff --git a/libutils/tests/Android.bp b/libutils/tests/Android.bp
index ea606a1..7cae133 100644
--- a/libutils/tests/Android.bp
+++ b/libutils/tests/Android.bp
@@ -23,6 +23,7 @@
     srcs: [
         "BitSet_test.cpp",
         "LruCache_test.cpp",
+        "Mutex_test.cpp",
         "Singleton_test.cpp",
         "String8_test.cpp",
         "StrongPointer_test.cpp",
@@ -72,6 +73,7 @@
         "-Wall",
         "-Wextra",
         "-Werror",
+        "-Wthread-safety",
     ],
 }