Inset the bubble bar expanded view instead of drawing our own header

* Use new method on TaskView to inset the app content when showing
  in bubble bar
* Update the visuals of the handle in the bubble bar expanded view to
  show three dots instead of a bar
* Make the header area a bit smaller (40 -> 32dp)
* Use a touch delegate to extend the touch area of the three dots
  past the top of the expanded view so that it's a 48dp height touch
  target.

Test: manual - enable bubble bar, get a bubble, expand it
             => observe the appearance of the header, should be 3 dots
                centered in the bar)
             => ensure that tapping slightly above the top edge of
                the expanded view triggers the menu
Bug: 279814257
Bug: 284205099
Change-Id: I2e918b1404d91b60c78e513ed0bf6d026109eb45