Iteration on findPath API. Address Jeff's comment in ag/1542773.

* Add @Nullable to rootId in Path constructor.
* Erase rootId if findPath() doesn't return null rootId.
* Use Log.wtf() instead of throwing.

Bug: 30948740
Change-Id: I38354c5ac49aaa7e31a3ae56cd3379ffb19918c9
(Cherry picked from commit 04fdf6e18c97c2bf6ed58f8de9ba6723fa8c1613)