a11y: Fixed screen reader issues with Kyle menus.

Screen readers no longer enter, exit, or report applications mode/region when
passing over menu trigger controls. Menu triggers can also be navigated
to with screen reader "jump to next/previous button" navigation shortcuts.

Fixes CNVS-15275

Test plan:
- Visit each kyle menu in Canvas.
- Verify with a screen reader:
  - That the trigger is reachable via normal arrow key navigation
  - That the trigger is reachable when navigating by buttons
  - That the menu opens when enter is pressed on the menu trigger
  - That forms mode is activated when the menu is opened (N/A in Voice Over).
  - That the up and down arrow keys allow one to navigate through the menu
  - That pressing enter on a menu option activates that action
- Verify that menu triggers are reachable by the keyboard (sans screen reader).
- Verify that all the other functionality relating to menus as discussed above
  is accessible to keyboard only users.

Change-Id: I81d7678251b7c43e67504d5569e59ccd2f846949
Reviewed-on: https://gerrit.instructure.com/40951
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Florence <ryanf@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>