Add a11y owners to //ash a11y feature subdirectories

autoclick, high_contrast, sticky_keys

NOTRY=true

Bug: none
Change-Id: I653502285779bf67bab6ecb80bb19fce05ca46cb
Reviewed-on: https://chromium-review.googlesource.com/933291Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Commit-Queue: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539158}

ash/autoclick/OWNERS

+file://ui/accessibility/OWNERS
+
+# TEAM: chromium-accessibility@chromium.org
+# COMPONENT: UI>Accessibility

ash/high_contrast/OWNERS

+file://ui/accessibility/OWNERS
+
+# TEAM: chromium-accessibility@chromium.org
+# COMPONENT: UI>Accessibility

ash/sticky_keys/OWNERS

-tengs@chromium.org
+file://ui/accessibility/OWNERS
 
-# COMPONENT: UI>Shell
+# TEAM: chromium-accessibility@chromium.org
+# COMPONENT: UI>Accessibility