Restructure public side of accessibility service.

This CL goes from this:
  //mojo/services/public/interfaces/accessibility

to this:
  //mojo/services/accessibility/public/interfaces

This CL also makes the Mojo-side changes necessary to roll this change into
Chromium (note that the accessibility service is not present in
mojo_services_public.gyp, but is depended on by html_viewer in the GN build in
Chromium).

TBR=beng

Review URL: https://codereview.chromium.org/791083002