Uninstall engine immediately and allow reinstall in onCancel.
Set previous list with default engine in onCancel.
Keep default engine change in operation and remove now unnecessary view refresh.
* For #5577 - Adds button to add a new search engine
* For #5577 - Adds custom engine store
* For #5577 - Creates a custom SearchEngineProvider
* For #5577 - Gives the ability to delete search engines
* For #5577 - Adds the UI to add a custom search engine
* For #5577 - Adds form to create a custom search engine
* For #5577 - Adds the ability to add a custom search engine
* For #5577 - Adds the ability to delete custom search engines
* For #5577 - Selects the first element on the add custom search engine screen
* For #5577 - Prevents adding a search engine that already exists
* For #5577 - Styles the add search engine preference
* For #5577 - Makes the name check case-insensitive
* For #5577 - Fix bug where home screen doesnt see new search engines
* For #5577 - Moves Search URL validation to its own type
* For #5577 - Fixes linting errors
* For #5577 - Adds the ability to edit a custom search engine
* For #5577 - Allows the user to edit a serach engine even when it is the last item in the list
* For #5577 - Adds an undo snackbar when deleting a search engine
* For #5577 - Moves all of the strings to be translated
* For #5577 - Fixes bug when deleting your default search engine
* For #5577 - Puts adding search engines behind a feature flag
* For #5577 - Navigate to custom search engine SUMO article when tapping learn more
* For #5577 - Fixes nits
* For #5577 - Uses concept-fetch to validate search string
* For #5577 - Adds string resources for the cannot reach error state
ekager