ui:preview Moved full screen toggle code to dedicated method.

Reduces duplicate code.

src/main/java/com/owncloud/android/ui/preview/PreviewImageFragment.java

@@ -188,13 +188,11 @@ public class PreviewImageFragment extends FileFragment {
         mImageView.setVisibility(View.GONE);
 
         view.setOnClickListener(v -> {
-            ((PreviewImageActivity) getActivity()).toggleFullScreen();
-            toggleImageBackground();
+            togglePreviewImageFullScreen();
         });
 
         mImageView.setOnClickListener(v -> {
-            ((PreviewImageActivity) getActivity()).toggleFullScreen();
-            toggleImageBackground();
+            togglePreviewImageFullScreen();
         });
 
         mMultiView = view.findViewById(R.id.multi_view);
@@ -784,6 +782,11 @@ public class PreviewImageFragment extends FileFragment {
         container.finish();
     }
 
+    private void togglePreviewImageFullScreen() {
+        ((PreviewImageActivity) getActivity()).toggleFullScreen();
+        toggleImageBackground();
+    }
+
     private void toggleImageBackground() {
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M && getFile() != null
                 && (getFile().getMimetype().equalsIgnoreCase(MIME_TYPE_PNG) ||