fix(slider): Change how step value is initialized (#1173)

[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.

(Sorry, had to recreate my previous pr)

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[codecov-io]

Codecov Report

Merging #1245 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1245   +/-   ##
======================================
  Coverage    99.9%   99.9%           
======================================
  Files          69      69           
  Lines        3307    3307           
  Branches      407     407           
======================================
  Hits         3304    3304           
  Misses          3       3

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

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[yeelan0319]

Thanks for the fix! 💯