fix(navBar): missing [disabled] scss selector

[rudzikdawid]

Closes #11268

PR Checklist

Please check that your PR fulfills the following requirements:

• The commit message follows our guidelines
• Tests for the changes have been added or this is not a bug fix / enhancement
• Docs have been added, updated, or were not required

PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Enhancement
[ ] Documentation content changes
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

https://material.angularjs.org/HEAD/demo/navBar
color of disabled button._md-nav-button is the same as normal enabled button: button.md-unselected

Issue Number:
#11268

What is the new behavior?

Color of disabled button should have value: '{{foreground-3}}'

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Before:

After:

[Splaktar]

LGTM