fix(top-app-bar): Change margin-top to padding-top to prevent margin collapse

[williamernest]

Fixes an issue with the fixed-adjust classes being susceptible to margin collapsing by changing margin-top to padding-top

[codecov-io]

Codecov Report

Merging #2643 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2643   +/-   ##
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  Coverage   98.48%   98.48%           
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  Files          98       98           
  Lines        4230     4230           
  Branches      537      537           
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  Hits         4166     4166           
  Misses         64       64

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[kfranqueiro]

Is the idea behind this to effectively "carry over" e.g. body margin so that it applies after / in addition to the adjustment? It took me a few minutes to understand why we would not want margin collapsing.

[williamernest]

Margin collapsing causes the top app bar to initially render 64px from the top of the page when using the mdc-top-app-bar--fixed-adjust class on a sibling element unless you use   to separate them.

[kfranqueiro]

I'm baffled at the bug this is fixing, but the fix seems OK.

[cheng93]

Media queries also need to changed

https://github.com/material-components/material-components-web/blob/master/packages/mdc-top-app-bar/_mixins.scss#L81