WB-1671: Dropdown: use `combobox` role in all openers

[jandrade]

Summary:

Addreses an issue in WB dropdowns where we were using the button role in the opener, which was causing issues with screen readers. This change updates the role to combobox that is more appropriate for the dropdown opener.

Issue: https://khanacademy.atlassian.net/browse/WB-1671

Test plan:

Navigate to the dropdowns in the Storybook and verify that the role of the opener is combobox.

More details TBD