feat(snackbar): add feature targeting for styles

[crisbeto]

Adds support for feature targeting to the mdc-snackbar styles.

Relates to #4227.

[crisbeto]

Here's a comparison between the before and after styles. A couple of lines had to be moved around so that they'd fit into the mixins.

before.css.txt
after.css.txt

[crisbeto]

Thank you for the feedback @abhiomkar, it should all be addressed now. Can you take another look?

[codecov-io]

Codecov Report

Merging #4876 into master will increase coverage by 0.3%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #4876     +/-   ##
=========================================
+ Coverage   98.86%   99.16%   +0.3%     
=========================================
  Files         119      119             
  Lines        5650     5650             
  Branches      749      749             
=========================================
+ Hits         5586     5603     +17     
+ Misses         63       46     -17     
  Partials        1        1

Impacted Files	Coverage Δ
packages/mdc-checkbox/component.ts	96.87% <0%> (+1.04%)	⬆️
packages/mdc-tab/component.ts	98.36% <0%> (+3.27%)	⬆️
packages/mdc-base/component.ts	100% <0%> (+3.57%)	⬆️
packages/mdc-ripple/component.ts	100% <0%> (+3.77%)	⬆️
packages/mdc-auto-init/index.ts	100% <0%> (+4.16%)	⬆️
packages/mdc-ripple/util.ts	97.56% <0%> (+4.87%)	⬆️
packages/mdc-switch/component.ts	98.27% <0%> (+6.89%)	⬆️
packages/mdc-radio/component.ts	95.83% <0%> (+8.33%)	⬆️

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