checkstyle: Add FallThrough

With bonus bugfix.

F-Droid/src/org/fdroid/fdroid/Preferences.java

@@ -280,6 +280,7 @@ public final class Preferences implements SharedPreferences.OnSharedPreferenceCh
                 for (ChangeListener listener : localRepoHttpsListeners) {
                     listener.onPreferenceChange();
                 }
+                break;
             case PREF_UNSTABLE_UPDATES:
                 for (ChangeListener listener : unstableUpdatesListeners) {
                     listener.onPreferenceChange();

config/checkstyle/checkstyle.xml

@@ -70,6 +70,7 @@
         <!--<module name="InnerAssignment" />-->
         <module name="DefaultComesLast" />
         <!--<module name="MissingSwitchDefault" />-->
+        <module name="FallThrough" />
 
         <module name="FinalClass" />