Fixed action button keyup also triggering click event

[Blackbaud-SteveBrush]

Addresses: #994

The keyup event also triggers a click event.

When the modal closes, it immediately focuses the trigger element. If the close button was triggered using the enter key, it will fire the keyup event on the newly focused trigger button. To avoid this behavior, we can instead register keydown on the action button, to prevent the close button's keyup from also being triggered.

To test, add an action button to the modal demo component and serve.

<sky-action-button (actionClick)="openModal('defaultModal')">
  <sky-action-button-header>
    Open modal with an action button
  </sky-action-button-header>
</sky-action-button>

[Blackbaud-SteveBrush]

@Blackbaud-BobbyEarl This is a much simpler approach to the previous pull request. Let me know your thoughts!

Old PR: #1255

[Blackbaud-SteveBrush]

Only change: using keydown instead of keyup.

[codecov-io]

Codecov Report

Merging #1448 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1448   +/-   ##
======================================
  Coverage     100%    100%           
======================================
  Files         382     382           
  Lines        7400    7399    -1     
  Branches      946     945    -1     
======================================
- Hits         7400    7399    -1

Impacted Files	Coverage Δ
...c/modules/action-button/action-button.component.ts	100% <100%> (ø)	⬆️

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