DropdownMenu and ActionMenu focus styles need background colors

[dgreif]

Describe the bug
DropdownMenu and ActionMenu both use keyboard shortcuts to navigate their ActionList Items. When focused, these items currently use the browser default outline. They should instead be using specific background coloring to indicate focus vs hover and not have an outline.

To Reproduce
Steps to reproduce the behavior:

1. Go to https://primer.style/components/DropdownMenu
2. Click on the demo dropdown
3. Press the down key
4. See outline instead of background color for focused item

Screenshots