.mdc-notched-outline__idle should be a sub-element of .mdc-notched-outline

[moog16]

What MDC Web Version are you using?

v0.32.0

What browser(s) is this bug affecting?

All

What OS are you using?

MacOSx 10.13.3

What are the steps to reproduce the bug?

1. open demo pages
2. look at notched outline html stucture

What is the expected behavior?

mdc-notched-outline__idle is a child of mdc-notched-outline

What is the actual behavior?

mdc-notched-outline__idle is sibling to mdc-notched-outline

Any other information you believe would be useful?

related PRs #2326 & #2401

[lynnmercier]

Somewhat related is #2415, which talks about calling Sass mixins for child components within a parent components stylesheet. Though 2415 might be a more general case than this bug.