Store picker - `Continue` button loading color in dark mode

[selanthiraiyan]

Description

The Continue button in the store picker screen is white in color during loading state even when dark mode is turned on.

Steps to reproduce

1. Install and launch the app.
2. Tap Skip to reach the login prologue screen.
3. Tap the Continue with WordPress.com button and sign in.
4. You will land on the store picker screen from where you can select a store.
5. Turn on dark mode and tap on the Continue button.
6. Observe that the Continue button is in white color during the loading state.

Expected behaviour

To maintain consistency, the Continue button should behave like the Continue button from the "Enter email" and "Enter password" screens.

Screenshots

Continue button in white color

Expected behaviour in Enter email screen
https://user-images.githubusercontent.com/524475/195509290-682ba8d0-c46f-4b9e-a79b-3fdb3410c217.MP4

Notes

I took a quick look at the code, and I noticed that the StorePickerViewController uses a custom button class FancyAnimatedButton. We should try to replace that with NUXButton from the Authenticator library to maintain the same behaviour in all screens in the login flow.