make background for exchange buttons black so they look

enabled/disabled when in fact they are.  Apparently now that the
theme's changed they don't look right without it, perhaps because the
old theme gave their layout a background other than white.

xwords4/android/XWords4/res/layout/board.xml

@@ -24,6 +24,7 @@
                 android:orientation="horizontal"
                 android:layout_width="fill_parent"
                 android:layout_height="wrap_content"
+                android:background="#FF000000"
                 >
     <Button android:id="@+id/exchange_commit"
             android:text="@string/button_trade_commit"