Animate when removing PiP

Fade out the PiP window when PipTaskOrganizer#removePip is called, which
covers the following scenarios:
- Tap on the close button
- Drag the PiP window to close area
- Disable PiP via settings

Note that when dragging to remove, we spring also the window off screen
and therefore, the fade out animation does not really appealing in that
case.

Video is taken with 10x normal duration.

Video: http://rcll/aaaaaabFQoRHlzixHdtY/gZfSf7SaGKepwiqvXA15dx
Bug: 159805747
Test: see video
Change-Id: Icc26c81711bae69bd00a2812661c00fd93055a92