Update Gaia API documents for #e615dc29592f1210bb86a0980bd917e965036054
    Bug 933167 - [Settings][V1.2] The settings for third party keyboards will still display after uninstalling the keyboard - r=arthur.chen

- Updated _initInstalledLayoutListView to use `ListView` of keyboards, within a `ListView` of layouts.
- Abstracted observing method to a helper for ListView provide easier detachment points