Support drag and drop from Taskbar

- Long clicking a BubbleTextView in Taskbar will start a system drag
  and drop operation, setting the original view invisible meanwhile.
- Defer gesture navigation when starting over a Taskbar item, and
  cancel any started gesture if a Taskbar drag and drop starts.

Bug: 171917176
Change-Id: If5049071fbf1755f545ee937daa4edabd869f00d