Add accessibility related notification into notification center

When chromevox is enabled or the braille is connected, a notification will be shown to
users through the notification center. The screenshot of the notifications can be found
in the crbug.

TEST=TrayAccessibilityTest.ShowNotification

BUG=685845

Review-Url: https://codereview.chromium.org/2838903002
Cr-Commit-Position: refs/heads/master@{#473939}
8 files changed