another minor pref tweak for mms fallback

1) fix summary for outgoing sms/mms
2) disable mms option when TS-users is unselected, same as the fallback
   approval option.

// FREEBIE

src/org/thoughtcrime/securesms/components/OutgoingSmsPreference.java

@@ -38,11 +38,16 @@ public class OutgoingSmsPreference extends DialogPreference {
     dataUsers.setOnClickListener(new View.OnClickListener() {
       @Override
       public void onClick(View v) {
-        askForFallback.setEnabled(dataUsers.isChecked());
+        updateEnabledViews();
       }
     });
 
+    updateEnabledViews();
+  }
+
+  private void updateEnabledViews() {
     askForFallback.setEnabled(dataUsers.isChecked());
+    neverFallbackMms.setEnabled(dataUsers.isChecked());
   }
 
   @Override