Remove overzealous mutate() sanity check in LayerDrawable

Child drawables of a LayerDrawable should be able to mutate() as a
no-op even if they !canConstantState.

Bug 7622311

Change-Id: I32c00961310f23aaf9ad868c419cbefb9d27a1d2

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

@@ -575,10 +575,6 @@ public class LayerDrawable extends Drawable implements Drawable.Callback {
     @Override
     public Drawable mutate() {
         if (!mMutated && super.mutate() == this) {
-            if (!mLayerState.canConstantState()) {
-                throw new IllegalStateException("One or more children of this LayerDrawable does " +
-                        "not have constant state; this drawable cannot be mutated.");
-            }
             mLayerState = new LayerState(mLayerState, this, null);
             final ChildDrawable[] array = mLayerState.mChildren;
             final int N = mLayerState.mNum;