Move the overflow ui to the bubble window

- An activity always runs on the main looper of the process
  and this simplifies threading since all bubble data will
  run on the shell thread
- The BubbleExpandedView will not initialize the task view
  when mIsOverflow is set.  We still snapshot the overflow
  view to keep the existing selection animation logic the
  same between overflow/task bubbles

Bug: 161979899
Test: atest WMShellUnitTests
Test: atest SytemUITests

Change-Id: If3632a3030c58c9ec334afcb06aac4c51e28938b