[fenix] Updating Fenix to work with the new logins API

[fenix] Updating Fenix to work with the new logins API

Switched to always using `Login` instead of the `SavedPassword` alias.

Made `MasterPasswordTipProvider.saveLogins()` call
`importLoginsAsync()`.  This is needed because it's the only method that
inputs a `Login` rather than a `LoginEntry`.

Moved the `SavedLoginsStorageController.kt.syncAndUpdateList` call
to inside `add()` and `update()`.  This simplifies the error handling a
bit.

Refactored dupe-checking code to use findLoginToUpdate()

Refactored `AddLoginFragment` / `EditLoginFragment` to put the username
error handling code all in 1 method.  I think it's easier to follow the
logic of showing/hiding the error labels when it's all in one place.
This fixes issues https://github.com/mozilla-mobile/fenix/pull/24103 and https://github.com/mozilla-mobile/fenix/pull/24104.  I would love to address https://github.com/mozilla-mobile/fenix/issues/24102,
but I'm not sure what the correct behavior is there so I just kept that
the same.