Use snapshot in enter content-pip animation

When entering content-pip
- We take a snapshot for the host task within the
  ActivityStarter#startActivityInner function, this is to make sure that
  the snapshot is taken before an app switches to the placeholder content
- We retrieve the snapshot in WM Shell when animating the newly created
  content-pip task, and use that snapshot during the animation
- This works similar to the solid color content overlay when app enters
  PiP without specifying a valid source rect hint. Therefore, we
  abstract the functionality to a dedicated PipContentOverlay class

Video: http://recall/-/aaaaaabFQoRHlzixHdtY/dRSTf7prHc0krPhTKZ0kXx
Bug: 220191201
Test: enter content-pip from ApiDemos app, see also Video
Change-Id: I2420402341a910b44bd8f64e3ad518ebef41bc19