Fix part #141 : Final UI for topic thumbnail (Image) with gradient over image (without topic-name).

app/src/main/java/org/oppia/app/databinding/DrawableBindingAdapters.kt

@@ -20,3 +20,10 @@ fun setBottomBackgroundDrawable(view: View, @ColorInt colorRgb: Int) {
   // The input color needs to have alpha channel prepended to it.
   (view.background as GradientDrawable).setColor((0xff000000 or colorRgb.toLong()).toInt())
 }
+
+@BindingAdapter("app:rectangleDrawableWithColor")
+fun setRectangleBackgroundDrawable(view: View, @ColorInt colorRgb: Int) {
+  view.setBackgroundResource(R.drawable.rectangle_background)
+  // The input color needs to have alpha channel prepended to it.
+  (view.background as GradientDrawable).setColor((0xff000000 or colorRgb.toLong()).toInt())
+}