Android: Manually detach the thread after calling quitApp.

On Android 5.0 ART will complain when we don't detach the thread before
the application is about to quit (non-fatal). This is because we
re-attach to call quitApp and then leave it attached.

Change-Id: I4571ef5f38d92afcaf91cb920ebe121a7be7835a
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@theqtcompany.com>

src/plugins/platforms/android/androidjnimain.cpp

@@ -443,6 +443,11 @@ static void *startMainMethod(void */*data*/)
     if (m_applicationClass)
         QJNIObjectPrivate::callStaticMethod<void>(m_applicationClass, "quitApp", "()V");
 
+    // All attached threads should be detached before returning from this function.
+    JavaVM *vm = QtAndroidPrivate::javaVM();
+    if (vm != 0)
+        vm->DetachCurrentThread();
+
     return 0;
 }