Android: nativeQuit for SDLActivity thread

- destroy Android_ActivityMutex
- display any SDL error message that may have occured in this thread,
  since SDL_GetError() is thread specific, and user has no access to it.

android-project/app/src/main/java/org/libsdl/app/SDLActivity.java

@@ -405,6 +405,8 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
             }
         }
 
+        SDLActivity.nativeQuit();
+
         super.onDestroy();
     }
 
@@ -727,6 +729,7 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
     public static native int nativeRunMain(String library, String function, Object arguments);
     public static native void nativeLowMemory();
     public static native void nativeSendQuit();
+    public static native void nativeQuit();
     public static native void nativePause();
     public static native void nativeResume();
     public static native void onNativeDropFile(String filename);

src/core/android/SDL_android.c

@@ -125,6 +125,9 @@ JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeLowMemory)(
 JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeSendQuit)(
         JNIEnv *env, jclass cls);
 
+JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeQuit)(
+        JNIEnv *env, jclass cls);
+
 JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativePause)(
         JNIEnv *env, jclass cls);
 
@@ -833,6 +836,25 @@ JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeSendQuit)(
     SDL_SemPost(Android_ResumeSem);
 }
 
+/* Activity ends */
+JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeQuit)(
+                                    JNIEnv *env, jclass cls)
+{
+    const char *str;
+
+    if (Android_ActivityMutex) {
+        SDL_DestroyMutex(Android_ActivityMutex);
+        Android_ActivityMutex = NULL;
+    }
+
+    str = SDL_GetError();
+    if (str && str[0]) {
+        __android_log_print(ANDROID_LOG_VERBOSE, "SDL", "SDLActivity thread ends (error=%s)", str);
+    } else {
+        __android_log_print(ANDROID_LOG_VERBOSE, "SDL", "SDLActivity thread ends");
+    }
+}
+
 /* Pause */
 JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativePause)(
                                     JNIEnv *env, jclass cls)