Only round the corners if the devices supports it

- In BubbleExpandedView the outline provider was using corner radius
  even if the device didn't support it
- In BubbleStackView the screenshot used when switching between bubbles
  also applied a corner radius even if the device didn't support it

This change ensures to check if rounded corners is supported before
applying it.

Test: manual - alter the code so that the device doesn't support rounded
               corners and check that expanding and switching between
               bubbles shows proper corners all the time
Fixes: 206902204
Change-Id: Ia0a8ceee825d4b399dd86f09306b021b38071e8d