android - overlap the shadow by a button

Question

This is what I currently have

This is what I want to achieve

Here is the xml layout

     <LinearLayout
         android:layout_width="wrap_content"
         android:id="@+id/contentLayout"
         android:orientation="vertical"
         android:gravity="top|center"
         android:background="@drawable/background_containernav"
         android:layout_height="wrap_content"
         android:padding="4dip" >

         <Spinner
             android:id="@+id/spnShapes"
             android:layout_height="wrap_content"
             android:layout_width="wrap_content" />

         <CheckBox
             android:text="Fill"
             android:id="@+id/chkFill"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content" />

         <Button
             android:id="@+id/btnPickColor"
             android:layout_width="wrap_content"
             android:layout_height="wrap_content"
             android:text="Color" />

     </LinearLayout>         

     <ImageView
         android:clickable="true"
         android:id="@+id/slideHandleButton"
         android:layout_width="wrap_content"
         android:scaleType="fitCenter"
         android:adjustViewBounds="true"
         android:src="@drawable/ic_buttonnav"
         android:layout_height="fill_parent"
         android:background="@drawable/background_buttonnav"/>

 </SlidingDrawer>

Here is the background_containernav.xml

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">
    <item>
        <shape xmlns:android="http://schemas.android.com/apk/res/android"
        android:shape="rectangle" >
            <solid android:color="#878787"/>
        </shape>   
    </item>   

    <item android:left="4px">
        <shape xmlns:android="http://schemas.android.com/apk/res/android"
        android:shape="rectangle" >
            <solid android:color="#fefefe"/>
        </shape>       
    </item>
</layer-list>

What should I do to achieve to the latter? Thank you very much.