Remove redundant memory barrier

pthread_create already includes the necessary memory barriers:
 - parent at pthread_create : pthread_mutex_unlock(start_mutex)
 - child at __thread_entry : pthread_mutex_lock(start_mutex)

Add lock around uses of mThread.

Added comments:
 - uses of mThread require lock
 - androidCreateRawThreadEtc returned ID is not safe for direct use from non-parent threads.

Change-Id: I18cb296b41ddaf64cf127b57aab31154319b5970
diff --git a/include/utils/threads.h b/include/utils/threads.h
index 41e5766..0bd69cf 100644
--- a/include/utils/threads.h
+++ b/include/utils/threads.h
@@ -526,6 +526,7 @@
     Thread& operator=(const Thread&);
     static  int             _threadLoop(void* user);
     const   bool            mCanCallJava;
+    // always hold mLock when reading or writing
             thread_id_t     mThread;
     mutable Mutex           mLock;
             Condition       mThreadExitedCondition;