This commit is a squash of 4 commits from #3281.

- Fix setting preferred color scheme logic for brave theme change
- Fix system dark mode change affect brave theme on Win10
    When brave theme is set to dark or light, system dark mode change
    should not affect brave theme.
- CleanUp: Delete SystemThemeSupportDarkMode
    Upstream added ui::NativeTheme::SystemDarkModeSupported().
    Replace ours with upstream one.
- Add dark mode change test by modifying reg value for Windows
    This test can verify whether theme change notification is delivered
    or not.
    If brave theme option is not default, system dark theme change should
    not trigger theme change notificagtion delivery.