Fix fling to dismiss behavior

Dismissing bubbles with fling is currently difficult on large screens, since it's hard to aim for the dismiss target.
This change increases the area around the dismiss target for larger screen width.

Fixes: 304180886
Test: Manual -- create bubbles and fling them to dismiss.

Change-Id: I1299265a67a893d72e4097bc99c20886a0694db6