fix(drawer): check for existence of ANIMATE class name in isOpening condition

[abhiomkar]

Fixes #4049

Programmatically trigger open / close drawer in this sequence:

Before implementation:

drawer.open = true;
// `isOpening()` returns `false` and class name `--closing` gets added
// before `--opening` in next animation frame. This causes drawer to be
// in weird state while it is still opening.
drawer.open = false;
drawer.open = true;

Changed the condition of isOpening to check for existence of both --animate and --opening classes to fix this.

[mdc-web-bot]

All 655 screenshot tests passed for commit a3253af vs. master! 💯🎉

[codecov-io]

Codecov Report

Merging #4078 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master    #4078   +/-   ##
=======================================
  Coverage   98.67%   98.67%           
=======================================
  Files         126      126           
  Lines        5603     5603           
  Branches      746      747    +1     
=======================================
  Hits         5529     5529           
  Misses         74       74

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

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[kfranqueiro]

LGTM with one more suggestion for tests

[mdc-web-bot]

All 663 screenshot tests passed for commit 5bec8d2 vs. master! 💯🎉

[mdc-web-bot]

All 663 screenshot tests passed for commit d0bcf2b vs. master! 💯🎉