Make ButtonWidgets more accessible

* Give them a role=button, and a default tabIndex of 0
* Listen for a keypress of 'space' and emit a click event
  (you'd have thought role=button would do this, but oh well)

Bug: 50047
Change-Id: I429ad165c95f34d26975daf81db18cc966802cde
Merge "Make ButtonWidgets more accessible"