Adding APIs to enable apps to influence accessibility traversal.

Accessibility focus is a token that can be put on any view and is
entirely controlled by accessibility services which decide whether
to put accessibility focus on a given view and what the traversal
strategy is.

Currently accessibility service use the view tree structure for
traversal which does not always reflect the logical order and the
user experience is sub-optimal. In such cases apps should be able
to influence how they are traversed for accessibility.

This change adds the notion of an accessibilty traversal order.
If a view has next in the traversal, an accessibility service has
to traverse the content of this view before traversing the content
next one.

This change will enable accessibility service to move away from
using the view tree and use spatial location of the content while
app developers can also influence the treversal order.

bug:17319923

Change-Id: Ib67ae4577f30b2c96575c71538dc1afdea08bfd3