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
Merge pull request #13364 from gnarf/933167

Bug 933167 - [Keyboard][V1.2] The settings value of third party keyboard will be kept if uninstalling the in used keyboard