Implement GradientDrawable getColorFilter.

Otherwise the value in setColorFilter is unavailable.

Bug:17137319
Change-Id: Ief973abfa129d935aa45336275337e19a3eb0f6a

graphics/java/android/graphics/drawable/GradientDrawable.java

@@ -810,6 +810,11 @@ public class GradientDrawable extends Drawable {
         }
     }
 
+    @Override
+    public ColorFilter getColorFilter() {
+        return mColorFilter;
+    }
+
     @Override
     public void setColorFilter(ColorFilter cf) {
         if (cf != mColorFilter) {