GitLab

- Adds an extension api, chrome.brailleDisplayPrivate.updateBluetoothBrailleDisplayAddress, which will [re]start brltty with the given device.
- adds a pref to track the braille display desired by a given user
- adds logic to start and stop brltty on launch given a bluetooth braille display

TODO:
- the UI which will do the initial connection/pairing, and unpairing/forgetting of a device

Bug: 882261
Test: manually call all relevant api functions.
Change-Id: Ief0189a3f76ea6518bd8b644d3df816435feb019
- call chrome.brailleDisplayPrivate.updateBluetoothBrailleDisplayAddress
- observe brltty is launched with the given address
- unload and reload chromevox and observe brltty is appropriately loaded/unloaded

Depends on:
https://chromium-review.googlesource.com/c/chromiumos/overlays/chromiumos-overlay/+/1308895

Change-Id: Ief0189a3f76ea6518bd8b644d3df816435feb019
Reviewed-on: https://chromium-review.googlesource.com/c/1308986
Commit-Queue: David Tseng <dtseng@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Reviewed-by: Dominic Mazzoni <dmazzoni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605185}