fix(checkbox): Implement component/adapter APIs to sync aria-checked

[kfranqueiro]

Fixes #2579.

[codecov-io]

Codecov Report

Merging #2580 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2580      +/-   ##
==========================================
+ Coverage   98.66%   98.66%   +<.01%     
==========================================
  Files          98       98              
  Lines        4197     4202       +5     
  Branches      534      534              
==========================================
+ Hits         4141     4146       +5     
  Misses         56       56

Impacted Files	Coverage Δ
packages/mdc-checkbox/foundation.js	97.63% <100%> (+0.05%)	⬆️
packages/mdc-checkbox/index.js	100% <100%> (ø)	⬆️

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