DropdownButton component doesn't allow a user to navigate back to the previously focusable element.

[TerminatorJonny]

The DropdownButton isn't accessible and doesn't allow a user to press 'shift + tab' in order to navigate to the previous element.

Steps:

1. Navigate to: DropdownButton
2. Click on an element before the Dropdown button (In order to get the focus to visually appear)
3. Press tab to the Dropdown button
4. Press 'shift + tab' to navigate back to the previous element.

Actual Results:
Once the dropdown button is in focus, the user is unable to navigate back to the previous element.

Expected Results:
The user should be able to navigate back to previous elements displayed on the page/view.