Fix cast exception from ScaleDrawable to ClipDrawble

We grab the progress drawable and cast it to ClipDrawable. This causes
a ClassCastException on phones where the manufacturer provides custom
view hierarchies for the slider but do not use the same classes as the
original Android views. However, the only method we call is defined on
Drawable, so this commit switches to using the Drawable superclass and
removes the cast to ClipDrawable.

Fixes #912

Change-Id: I20b0691ca684a2e34140aafa9d3b04eb4adf7148

appinventor/components/src/com/google/appinventor/components/runtime/Slider.java

@@ -2,7 +2,7 @@ package com.google.appinventor.components.runtime;
 
 import android.R;
 import android.graphics.PorterDuff;
-import android.graphics.drawable.ClipDrawable;
+import android.graphics.drawable.Drawable;
 import android.graphics.drawable.LayerDrawable;
 import android.util.Log;
 import android.view.View;
@@ -59,7 +59,7 @@ public class Slider extends AndroidViewComponent implements SeekBar.OnSeekBarCha
   // the total slider width
   private LayerDrawable fullBar;
   // the part of the slider to the left of the thumb
-  private ClipDrawable beforeThumb;
+  private Drawable beforeThumb;
 
   // colors of the bar after and before the thumb position
   private int rightColor;
@@ -97,7 +97,8 @@ public class Slider extends AndroidViewComponent implements SeekBar.OnSeekBarCha
     seekbar = new SeekBar(container.$context());
 
     fullBar = (LayerDrawable) seekbar.getProgressDrawable();
-    beforeThumb = (ClipDrawable) fullBar.findDrawableByLayerId(R.id.progress);
+    beforeThumb = fullBar.findDrawableByLayerId(R.id.progress);
+
     leftColor = initialLeftColor;
     rightColor = initialRightColor;
     setSliderColors();