Iteration on findPath API.

* Extend this API to take tree URI
* Add toString(), equals() and hashCode() to Path
* Address Jeff's comments in ag/1513538
* Add unit tests for findPath

Bug: 30948740
Change-Id: Iaf852d0e40fae37623e9bb9ffa1c6fbe334c1b21
(cherry picked from commit d4ab7ade7171a4382ef4f61f2a5f078a17800e83)