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
021078554b