part of #8088
Just the part where a playing video minimizes to corner when scrolling up

• add experiment flag
• add attribute dock=(when-playing|never) (default to never for now)
• restricted to phone/portrait mode only for now.
• works/integration tested for amp-video and amp-youtube

does NOT include

• any controls or gestures to dismiss, etc..
• different UX based on scroll direction
• configurable docking location