override onResume not onStart to fix NPE

When game's resumed (brought from background) and Wait alert is up
onStart() is called before getDialog() has a non-null value to
return. onResume() is the place to modify the dialog.

xwords4/android/app/src/main/java/org/eehouse/android/xw4/XWDialogFragment.java

@@ -47,9 +47,9 @@ class XWDialogFragment extends DialogFragment {
     }
 
     @Override
-    public void onStart()
+    public void onResume()
     {
-        super.onStart();
+        super.onResume();
 
         if ( null != m_buttonMap ) {
             AlertDialog dialog = (AlertDialog)getDialog();