activities: Added @Override annotation above the onDestroy() method signature.

Using the @Override annotation is useful for two reasons:

It elicits a warning from the compiler if the annotated method doesn't actually override anything, as in the case of a misspelling.
It improves the readability of the source code by making it obvious that methods are overridden.

src/main/java/com/owncloud/android/ui/activities/ActivitiesActivity.java

@@ -149,6 +149,7 @@ public class ActivitiesActivity extends FileActivity implements ActivityListInte
         refreshLayout.setColorSchemeColors(accentColor, primaryColor, darkColor);
     }
 
+    @Override
     public void onDestroy() {
         super.onDestroy();
         unbinder.unbind();