Bubble dismiss updates

- large colored circle, uses same accent color that launcher
  has for remove / uninstall
- added a shrink animation for the target when the bubble is
  stuck via the magnet
- no longer desaturate the bubble being dismissed but rather add
  a bit of an alpha

Bug: 183657564
Test: manual - get a bubble, drag it around & dismiss it, notice
               the animation
Change-Id: If0d0d86b7389f0fa3314598266cd83ecabdc6a9b