This patch includes 4 main changes
1. Created a guard in `removeLaunchOnLogin` with `getLaunchOnLoginApproved` to make sure the setting is not already disabled before removing the registry key made by Firefox.
2. Change the method doc for `removeLaunchOnLogin` to better describe why we need to guard the function and provide necessary context.
3. Prefixed the functions called within `removeLaunchOnLogin` with "_", to signify that they should not be used as a public API. We should only call those functions after we've checked the return value of `getLaunchOnLoginApproved`.
4. Modified existing unit test, and added a new unit test to make sure `removeLaunchOnLogin` is non-op when Windows has disabled such feature.
Differential Revision: https://phabricator.services.mozilla.com/D227399
4bced98a4c