Adding explicit text traversal granularities and actions for web navigation.

1. The granularities for traversing the text content of an accessibility
   node info are now predefined constants and custom ones will not be
   supported. This is the simplest solution - we can always add namespaced
   user defined ones (unlikely).

2. Added actions for traversing web content. These actions can be used by
   an accessibility service to transparently drive the JavaScript based
   screen reader that is used for handling web content.

3. Added a new accessibility event type for traversing the content of a
   view. This event is needed to announce to the user what is the next
   element, i.e. the one next to the cursor, after the view's text was
   traversed.

bug:5932640
bug:6389591

Change-Id: I144647da55bc4005c64f89865ef333af8359e145