Adding APIs to enable reporting virtual view hierarchies to accessibility serivces.

Added an interface that is the contract for a client to expose a virtual
view hierarchy to accessibility services. Clients impement this interface
and set it in the View that is the root of the virtual sub-tree. Adding
this finctionality via compostion as opposed to inheritance enables apps
to maintain backwards compatibility by setting the accessibility virtual
hierarchy provider on the View only if the API version is high enough.

bug:5382859

Change-Id: I7e3927b71a5517943c6cb071be2e87fba23132bf