Bug 912010 - [Keyboard][V1.2] Default Keyboard when all keyboards are de-selected - r=rlu r=garychen r=arthurcc

- Refactors `KeyboardHelper`
  - Renames the application method to `getApps`, which now caches response until the apps change
  - Adds a new API `getLayouts` to get a callback with an array of layouts.
  - Adds a new API `watchLayouts` to get a callback whenever layout settings or apps change
  - No longer calls `saveToSettings` itself when changing layout parameters.  This allows a consumer to batch operations before saving.
  - New method `checkDefaults` will make sure there is at least one `text` `url` and `number` keyboard enabled, calling the callback with anything it enabled.
  - +unit test suite.

- ManifestHelper now automatically returns a ManifestHelper for each object it contains
  - +unit test suite
  - now waits until access time to handle the language selection instead of creation time
     - this allows holding the one manifest helper and having its "name" property change with the language

- Settings Changes
  - Now uses `watchLayouts` from keyboard_helper
  - Now calls `checkDefaults` when disabling a keyboard
    - If a default is enabled opens a DefaultKeyboardEnabledDialog
  - Removed a bunch of commented out code for dealing with the old layout settings

- System Changes
  - Now uses `watchLayouts` from keyboard_helper
  - Will call `checkDefaults` whenever it can't find a keyboard to show, showing the default from `text` if it still can't find one

- Test suite
  - Moved some mocks to shared and updated require statements
  - Mock Settings now allows a "sync mode" that can call the callbacks for observers on demand.

Closes gh-12866