Minor UI adjustments to a11y button chooser

- Allow scrim-touch or downward-drag to dismiss the chooser
- Ensure the bottom drawer takes up only as much space as it needs
  to prevent it from being draggable

Bug: 36871827
Test: Manual - verified appearance and behavior changes
Change-Id: I1779814a7ee386f4a32faed5c0eb799ebe4706e5