input_decorator should use boolean and render widgets directly instead of using `_helper` and `_error` cached widgets

The code around showing the `_helper` and `_error` widget seem weird. Investigate and try and see if we can just build the widgets directly instead of caching them.

Context:
https://github.com/flutter/flutter/blob/43678b397ff4bba38a9e6466f6f9b317563fc948/packages/flutter/lib/src/material/input_decorator.dart#L322