fix(ripple): Move feature detect CSS to mixins

[kfranqueiro]

This allows the feature detect CSS to be automatically included by other components
that rely on ripple mixins (e.g. button). This problem was evidenced after I split demos.scss in #1285 which caused regressions in Edge due to the feature-detect CSS no longer being loaded in other pages such as button.html. Thanks @yeelan0319 for catching this!

It may seem odd for a mixins file to emit styles, but ripple's mixins file already does so because it imports ./keyframes (which contributes to #1295). In both cases (keyframes and the feature-detect styles), this is done so that other components which are designed to support or instantiate ripples (e.g. button, dialog, etc.) function correctly without needing to separately explicitly include mdc-ripple.scss.

[codecov-io]

Codecov Report

Merging #1302 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1302   +/-   ##
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  Coverage    99.9%   99.9%           
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  Files          69      69           
  Lines        3314    3314           
  Branches      409     409           
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  Hits         3311    3311           
  Misses          3       3

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[lynnmercier]

LGTM...given that we already do this for Ripple, and it fixes a regression.

But in the future we really need to revisit this Ripple code, and its relationship to other components.