Android: don't allow multiple instance of SDLActivity

Default launch mode (standard) allows multiple instances of the SDLActivity.
( https://developer.android.com/guide/topics/manifest/activity-element#lmode )

Not sure this is intended in SDL as this doesn't work. There are static
 variables in Java, in C code which make this impossible (allow one android_window) and
 also Audio print errors.

There is also some code added in onDestroy as if it would be able to
re-initialize: https://hg.libsdl.org/SDL/rev/27686adb08c3

Bug Android activity life-cycle seems to show there is not transition to get out
of onDestroy()
https://developer.android.com/reference/android/app/Activity#ActivityLifecycle

( can be tested with "adb shell am start  my.package.org/.MainActivity"
  and "adb shell am start -n  my.package.org/.MainActivity" )

Send me a message if there are real use-case for this !

android-project/app/src/main/AndroidManifest.xml

@@ -54,6 +54,8 @@
      
         <activity android:name="SDLActivity"
             android:label="@string/app_name"
+            android:alwaysRetainTaskState="true"
+            android:launchMode="singleInstance"
             android:configChanges="keyboard|keyboardHidden|orientation|screenSize"
             >
             <intent-filter>

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

@@ -386,8 +386,6 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
 
         if (SDLActivity.mBrokenLibraries) {
            super.onDestroy();
-           // Reset everything in case the user re opens the app
-           SDLActivity.initialize();
            return;
         }
 
@@ -405,15 +403,9 @@ public class SDLActivity extends Activity implements View.OnSystemUiVisibilityCh
             } catch(Exception e) {
                 Log.v(TAG, "Problem stopping thread: " + e);
             }
-            SDLActivity.mSDLThread = null;
-
-            //Log.v(TAG, "Finished waiting for SDL thread");
         }
 
         super.onDestroy();
-
-        // Reset everything in case the user re opens the app
-        SDLActivity.initialize();
     }
 
     @Override
@@ -1711,12 +1703,12 @@ class SDLSurface extends SurfaceView implements SurfaceHolder.Callback,
            return;
         }
 
-        /* Surface is ready */
-        SDLActivity.mIsSurfaceReady = true;
-
         /* If the surface has been previously destroyed by onNativeSurfaceDestroyed, recreate it here */
         SDLActivity.onNativeSurfaceChanged();
 
+        /* Surface is ready */
+        SDLActivity.mIsSurfaceReady = true;
+
         SDLActivity.mNextNativeState = SDLActivity.NativeState.RESUMED;
         SDLActivity.handleNativeState();
     }