fix(top-app-bar): Replace margin-top in media query with padding-top

[williamernest]

Missed a change to mobile fixed-adjust class with #2643

Resolves #2712.

[codecov-io]

Codecov Report

Merging #2704 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2704   +/-   ##
=======================================
  Coverage   98.48%   98.48%           
=======================================
  Files          98       98           
  Lines        4230     4230           
  Branches      537      537           
=======================================
  Hits         4166     4166           
  Misses         64       64

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[alex-page]

If anyone is experiencing this issue and needs a temporary patch they can do this:

@media (max-width: 599px){
	.mdc-top-app-bar--fixed-adjust {
		margin-top: 0;
		padding-top: $mdc-top-app-bar-mobile-row-height;
	}
}