Draw horizontal line on bottom of drawable programmatically

Question

I have to change the divider color of my listView dynamically.

1. solution:
   Therefore I to create a Drawable programmatically that draws a horizontal line and set it as background of my convertView. Now the line is in the middle of my convertView. How can I align it to the bottom?

   public class DividerView extends Drawable {
   private Paint mPaint;
   
   public DividerView() {
       mPaint = new Paint();
       mPaint.setStrokeWidth(3);
   }
   
   @Override
   public void draw(Canvas canvas) {
       int lvl = getLevel();
       Rect b = getBounds();
       float x = b.width() * lvl / 10000.0f;
       float y = (b.height() - mPaint.getStrokeWidth()) / 2;
       mPaint.setColor(Color.BLACK);
       canvas.drawLine(x, y, b.width(), y, mPaint);
   }
   
   @Override
   protected boolean onLevelChange(int level) {
       invalidateSelf();
       return true;
   }
   
   @Override
   public void setAlpha(int alpha) {
   }
   
   @Override
   public void setColorFilter(ColorFilter cf) {
   }
   
   @Override
   public int getOpacity() {
       return PixelFormat.TRANSLUCENT;
   }
   }
   

2. solution:
   I also tried to use a shape drawable as divider and change its color. But I get the following exception:

   LayerDrawable cannot be cast to android.graphics.drawable.GradientDrawable
   

How can I access the stroke in my layer-list?
Here is my code:

divider.xml

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

<item>
    <shape android:shape="rectangle" >
        <solid android:color="@android:color/transparent" />
    </shape>
</item>

<item android:top="-6dp" android:right="-6dp" android:left="-6dp">
    <shape>
        <solid android:color="@android:color/transparent" />
        <stroke
            android:width="1dp"
            android:color="#000000" />
    </shape>
</item>

ListAdapter.java

GradientDrawable drawable = (GradientDrawable)convertView.getBackground();
                    drawable.setStroke(3, Color.RED);
                    convertView.setBackground(drawable);