Navbar color and custom CSS clash on login page.

This is a follow on from MOBILE-2959 and  MOBILE-3004

As discussed on those tickets, some platforms let you color the "status bar" area at the very top of the phone and currently the Moodle app will use orange for this purpose, until you enter a site with a CSS style applied, at which point it turns black.

There's a small issue remaining though. Just before you log into such a site, the login screen fetches the colors and potentially changes the look of the login screen, but the orange status bar is still visible at the top.

Possibly it makes more sense to make the switch to black as soon as the site styles are detected during login, rather than waiting for the next page.