fix(overlays): focus management with checkbox/radio

Issue number: resolves internal

What is the current behavior?

Using Tab or Shift + Tab to focus through elements in a modal won't behave as expected when using ion-checkbox or ion-radio within an ion-item. Previously, the behavior would result in the last item in a list getting focus styling, but document.activeElement would still be the first actionable item in the overlay

What is the new behavior?

For checkboxes, the ion-checkbox element itself will be focused rather than the encapsulating ion-item

For radios, the ion-radio-group will be used to focus the appropriate element. This will be the first ion-radio if there is no "checked" item, or the "checked" item if one exists.

Does this introduce a breaking change?

[ ] Yes
[x] No

ionic-team / ionic-framework