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>