Fixing Drawable methods for find by layer ID and invalidation callback

BUG: 12475487, 12475575
Change-Id: Icbf11c76eaf9357fe2076bf60f7e448db65bc835

graphics/java/android/graphics/drawable/LayerDrawable.java

@@ -229,15 +229,18 @@ public class LayerDrawable extends Drawable implements Drawable.Callback {
     }
 
     /**
-     * Look for a layer with the given id, and returns its {@link Drawable}.
+     * Looks for a layer with the given ID and returns its {@link Drawable}.
+     * <p>
+     * If multiple layers are found for the given ID, returns the
+     * {@link Drawable} for the matching layer at the highest index.
      *
      * @param id The layer ID to search for.
-     * @return The {@link Drawable} of the layer that has the given id in the hierarchy or null.
+     * @return The {@link Drawable} for the highest-indexed layer that has the
+     *         given ID, or null if not found.
      */
     public Drawable findDrawableByLayerId(int id) {
         final ChildDrawable[] layers = mLayerState.mChildren;
-        final int N = mLayerState.mNum;
-        for (int i = 0; i < N; i++) {
+        for (int i = mLayerState.mNum - 1; i >= 0; i--) {
             if (layers[i].mId == id) {
                 return layers[i].mDrawable;
             }
@@ -370,7 +373,7 @@ public class LayerDrawable extends Drawable implements Drawable.Callback {
     }
 
     /**
-     * Invalidates cached padding.
+     * Invalidates cached padding and recomputes child bounds.
      */
     private void invalidatePadding() {
         mHasCachedPadding = false;
@@ -379,10 +382,9 @@ public class LayerDrawable extends Drawable implements Drawable.Callback {
 
     @Override
     public void invalidateDrawable(Drawable who) {
-        final Callback callback = getCallback();
-        if (callback != null) {
-            callback.invalidateDrawable(this);
-        }
+        // Something changed, maybe it was the child drawable's padding.
+        invalidatePadding();
+        invalidateSelf();
     }
 
     @Override