Desktop inputs require `width: null` to flex

[TomTirapani]

We have a pattern in Desktop inputs, where we specify a default width in the style props, e.g.

width: withDefault(width, 200),
flex: withDefault(flex, null)

Unfortunately, this means that a dev has to pass width: null in order for flex: true to take effect, since the set width takes precedence.

We could consider conditionally applying the default width only if flex is nil.