Fixed Android save export 'corruption'

This was caused by downright atrocious default Android API behavior. Read https://issuetracker.google.com/issues/180526528?pli=1

android/src/mindustry/android/AndroidLauncher.java

@@ -160,7 +160,7 @@ public class AndroidLauncher extends AndroidApplication{
                                     @Override
                                     public OutputStream write(boolean append){
                                         try{
-                                            return getContentResolver().openOutputStream(uri);
+                                            return getContentResolver().openOutputStream(uri, "rwt");
                                         }catch(IOException e){
                                             throw new ArcRuntimeException(e);
                                         }