Enables drag to dismiss for PiP and adds a dark scrim when dismissing

- Adds a scrim that fades in as the PiP is dragged past the bottom movement
  bounds.
- Tweaks the values for minimizing the PiP and dismissing the PiP
- Fixes an issue where the PiP could be minimized in the bottom corners
  of the screen

Test: Manual - drag PiP to bottom of screen to dismiss or swipe down on
PiP when it's at the bottom of the screen
Bug: 35358768
Bug: 35358628
Change-Id: Ib4ad5ed7094fef76ea979b46ea7e03d746625d00