Handle swipe down on taskbar to stash

- Moved all touch-to-stash logic to new TaskbarStashViaTouchController
  (handles both tap outside to stash instantly as well as swipe down
  inside to stash after letting go)
- This is a TouchController on TaskbarDragLayer, so it intercepts
  touches from TaskbarView before icons can be dragged during swipe down

Test: swipe up to invoke transient taskbar in an app, swipe down or
touch outside to stash
Flag: ENABLE_TRANSIENT_TASKBAR=true
Fixes: 246631710

Change-Id: I5cf64848bba34ad32fcc80a93fb4f79ebd2c10a7