fix(slider): Change how step value is initialized

[archnode]

resolves #1173

Micro-change, but the "setStep"-function actually triggers a "setValue" which sets aria-valuenow to null since the value is still uninitialized at this time.

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[archnode]

I signed, I signed ..

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[codecov-io]

Codecov Report

Merging #1182 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1182   +/-   ##
======================================
  Coverage    99.9%   99.9%           
======================================
  Files          69      69           
  Lines        3306    3306           
  Branches      406     406           
======================================
  Hits         3303    3303           
  Misses          3       3

Impacted Files	Coverage Δ
packages/mdc-slider/foundation.js	100% <100%> (ø)	⬆️

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[lynnmercier]

This generally looks good, but please add a test to cover this case.

And @archnode, our Googlebot still wants you to sign the CLA

[lynnmercier]

Please add a test for this case