[TextField] Fixes underline custom styles not inherited by focusedUnderline.

[newoga]

This fixes an issue introduced in PR #2476. I noticed in the documentation in Divider.jsx that the TextField underline were being hidden using style prop using display: 'none'. The underline was invisible when it was not focused, however when it was focused, the underline became visible. The issue was that custom styles passed to underline were not inherited by the focusedUnderline which is used when the underline has an error or is focused. Now it does (as demonstrated in lines L111-112).

Before (L111-L112):

-  let underline = Styles.merge(styles.root, style);
-  let focusedUnderline = Styles.merge(styles.root, styles.focus, focusStyle);

After (L111-L112):

+  let underline = Styles.merge(styles.root, style);
+  let focusedUnderline = Styles.merge(underline, styles.focus, focusStyle);

I also made the following minor code formatting changes:

1. Changed Styles to styleUtils (a convention that we defined in the Divider.jsx pull request)
2. Change styles variable from let to const (L84)

[oliviertassinari]

@newoga Nice catch, thanks!