Bug 1774285 - Avoid a11y instantiation after clipboard copy. r=nlapre,jamie, a=dmeehan

This prevents a11y from getting instantiated shortly after clipboard paste, in
order to prevent hangs with the Windows 11 suggested actions feature.

When combined with the previous patch, the behavior is the following:

 * For users with a11y already-enabled:

   * No hang (due to clipboard flush).
   * Quick actions menu is positioned at selection offset.

 * For users with a11y disabled (most):

   * No hang (due to no a11y instantiation + clipboard flush).
   * Quick actions menu is positioned at pointer (cursor) offset.

Co-Authored-By: Emilio Cobos Álvarez <emilio@crisal.io>

Differential Revision: https://phabricator.services.mozilla.com/D160652

Depends on D160646