SoundPool: Fix StreamManager JavaThread shutdown

std::future destructor only blocks for ready when created by std::async,
so do a wait in the JavaThread destructor.

Do not signal that the thread is closed until we really are finished
with all member variable access.

This fixes a rare race condition.

Test: SoundPool stress test
Test: SoundPoolAacTest SoundPoolHapticTest SoundPoolMidiTest SoundPoolOggTest
Bug: 150517918
Change-Id: I412143726956ee100069f711dd39b1d59ac100b0

media/jni/soundpool/StreamManager.h

@@ -52,6 +52,12 @@ public:
 
     JavaThread(JavaThread &&) = delete; // uses "this" ptr, not moveable.
 
+    ~JavaThread() {
+        join(); // manually block until the future is ready as std::future
+                // destructor doesn't block unless it comes from std::async
+                // and it is the last reference to shared state.
+    }
+
     void join() const {
         mFuture.wait();
     }
@@ -64,8 +70,9 @@ private:
     static int staticFunction(void *data) {
         JavaThread *jt = static_cast<JavaThread *>(data);
         jt->mF();
-        jt->mIsClosed = true;
         jt->mPromise.set_value();
+        jt->mIsClosed = true;  // publicly inform that we are closed
+                               // after we have accessed all variables.
         return 0;
     }