Make bubbles slightly more accessible; provide actions

* Bubbles can be focused
* Accessibility actions added for:
   - dismissing stack
   - expanding stack
   - collapasing stack

There will be more work to do here with content descriptions and
individual bubble actions.

Bug: 123541035
Fixes: 126284597
Test: manual 1) turn on talkback and open local context menu while focusing
               a bubble; note that dismiss / expand are available
             => ensure expand and dismiss from this state do whats expected
             2) from expanded state open local context menu; note that
                dismiss / collapse are available
             => ensure collapse and dismiss from this state do whats expected
Change-Id: I259d34d3fa78ad07f55fc64767b72382d023c2ab