Add consistent @NonNull annotations for drawable callbacks

Bug: 27599515
Change-Id: I33fdc5392302403bfff9cc74a8226173ec962af6
diff --git a/core/java/android/widget/CompoundButton.java b/core/java/android/widget/CompoundButton.java
index b19fe17..5d7585f 100644
--- a/core/java/android/widget/CompoundButton.java
+++ b/core/java/android/widget/CompoundButton.java
@@ -474,7 +474,7 @@
     }
 
     @Override
-    protected boolean verifyDrawable(Drawable who) {
+    protected boolean verifyDrawable(@NonNull Drawable who) {
         return super.verifyDrawable(who) || who == mButtonDrawable;
     }