only reload timeline on pref change when necessary

app/src/main/java/com/keylesspalace/tusky/adapter/TimelineAdapter.java

@@ -98,6 +98,10 @@ public final class TimelineAdapter extends RecyclerView.Adapter {
         mediaPreviewEnabled = enabled;
     }
 
+    public boolean getMediaPreviewEnabled() {
+        return mediaPreviewEnabled;
+    }
+
     @Override
     public long getItemId(int position) {
         return dataSource.getItemAt(position).getViewDataId();

app/src/main/java/com/keylesspalace/tusky/fragment/TimelineFragment.java

@@ -610,8 +610,11 @@ public class TimelineFragment extends SFragment implements
             }
             case "mediaPreviewEnabled": {
                 boolean enabled = sharedPreferences.getBoolean("mediaPreviewEnabled", true);
-                adapter.setMediaPreviewEnabled(enabled);
-                fullyRefresh();
+                boolean oldMediaPreviewEnabled = adapter.getMediaPreviewEnabled();
+                if(enabled != oldMediaPreviewEnabled) {
+                    adapter.setMediaPreviewEnabled(enabled);
+                    fullyRefresh();
+                }
                 break;
             }
             case "tabFilterHomeReplies": {