fix(input): underline should only be thicker if focused

[devversion]

• Currently the underline of the input container is always 2px thick if the input is invalid. As per specifications the underline should be 1dp/px if not focused.

@mmalerba The specs are not too clear about the intended behavior here. There are sections about the thickness of the underline, but not specifically in relation to error handling.

References #6140

[mmalerba]

I actually made this change intentionally (#4506) I agree that the spec isn't 100% clear, but I think they intended to make error state 2px always (see summary chart). Actually according to the chart hover state should be 2px too, which it currently isn't.

As for #6140, yes the select and the input currently differ in a number of ways that makes them look really bad when placed side by side. We need to refactor them to use the same CSS, I would like to use the CSS for the input since I've spent a lot of time making sure that it matches the spec relatively well.

[mmalerba]

Ok given that the official spec is kind of vague on this I'm ok with changing it back to think only on focus. I think its better from an a11y standpoint for users to be able to visually distinguish focused from unfocused state.

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