请教下android里如何设置gridview里scrollbar的位置呢？

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/semitransparent"
    android:orientation="vertical" >    <GridView
        android:id="@+id/channel_List_gridview"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_marginLeft="30dp"
        android:layout_marginRight="30dp"
        android:layout_marginTop="30dp"
        android:alwaysDrawnWithCache="true"
        android:scrollbars="vertical"
        android:columnWidth="80dp"
        android:gravity="center"
        android:padding="30dip"
        android:listSelector="@drawable/channel_list_sl"
        android:numColumns="5"
        android:scrollbarThumbVertical="@drawable/scroll_bgcolor1"
android:scrollbarTrackVertical="@drawable/scroll_bgcolor"
        android:verticalSpacing="45dp" 
        android:horizontalSpacing="45dp"
        android:fadeScrollbars="false"
        />
</RelativeLayout>scroll_bgcolor.xml
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient android:startColor="#032C2F" android:endColor="#032C2F"
            android:angle="0"/>
    <corners android:radius="0dp" />
    <padding android:bottom="50dp"/>
</shape>scroll_bgcolor1.xml
<?xml version="1.0" encoding="utf-8"?>
<shape xmlns:android="http://schemas.android.com/apk/res/android">
    <gradient android:startColor="#00FFFF" android:endColor="#00FFFF"
            android:angle="0"/>
    <corners android:radius="6dp" />
</shape>
在网上查说是scrollbarThumbVertical这个可以设定滚动条的短条的长度
            scrollbarTrackVertical这个可以设定滚动条背景长条的长度
但是设置了<padding android:bottom="50dp"/>后并未有任何的改变，依然没有改变滚动条离底部的距离，请问该怎么解决呢？？