cros: move ShowAccessibilityNotification to A11yController

changes:
Remove ShowAccessibilityNotification in AccessibilityObserver. That is
only used in TrayAccessibility, while TrayAccessibility should be pure
views code. Move the function and tests to AccessibilityController.

Bug: 800270
Test: covered by tests
Change-Id: I0551c8b36d1a4bfb030b786bd78d610627113828
Reviewed-on: https://chromium-review.googlesource.com/1007891
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Qiang Xu <warx@google.com>
Cr-Commit-Position: refs/heads/master@{#551253}
7 files changed