[flutter_local_notifications_linux] fixed API docs by changing stubbing approach

[MaikuB]

This fixes an issue on how the API docs for the stub implementation were shown as the docs are generated on a platform that doesn't support FFI. This is resolved by using conditional imports to determine if a stub implementation of the NotificationManager referenced instead

This PR also includes some other fixes to API docs and updated the file name of the NotificationManager class and correspond test file