Support 9-patch drawable for spacebar icon

        <!-- Background image for the spacebar.  This image needs to be a

             {@link StateListDrawable}, with the following possible states: normal, pressed. -->

        <attr name="spacebarBackground" format="reference" />

        <attr name="spacebarIconWidthRatio" format="float" />

        <!-- Horizontal padding of left/right aligned key label to the edge of the key. -->

        <attr name="keyLabelHorizontalPadding" format="dimension" />

        <!-- Right padding of hint letter to the edge of the key.-->

            <!-- This should be aligned with Key.LABEL_FLAGS__* -->

            <flag name="alignLeft" value="0x01" />

            <flag name="alignRight" value="0x02" />

            <flag name="alignButtom" value="0x04" />

            <flag name="alignLeftOfCenter" value="0x08" />

            <flag name="fontNormal" value="0x10" />

            <flag name="fontMonoSpace" value="0x20" />