Organize files under ash/accessibility

Pure refactor with only pathschanged.

R=josiahk@google.com, xiyuan@chromium.org

AX-Relnotes: n/a
Change-Id: Ib30ad834a32147d348042c3ae7094d41712e0543
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2783623


Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: David Tseng <dtseng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#866184}