Fix LayerDrawable#mutate

Change-Id: I985b80141ffc96db969eaf36538df938cb093661

Author: adamp

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

@@ -575,6 +575,11 @@ public ConstantState getConstantState() {
     @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;
             for (int i = 0; i < N; i++) {
@@ -694,7 +699,7 @@ public final boolean isStateful() {
             return stateful;
         }
 
-        public synchronized boolean canConstantState() {
+        public boolean canConstantState() {
             if (!mCheckedConstantState && mChildren != null) {
                 mCanConstantState = true;
                 final int N = mNum;