Tweaks to ViewOverlay.

* Clarify Javadocs for add / remove when called with the same
Drawable.
* Add @NonNull annotation to all add / remove APIs and throw
IllegalArgumentException when null value is passed.

Bug: 27528798
Change-Id: Ied8f28c70775309e4fa85aff6a7202c1a0eb6aa3
2 files changed