part of #8088 Just the part where a playing video minimizes to corner when scrolling up - [x] add experiment flag - [x] add attribute `dock=(when-playing|never)` (default to never for now) - [x] restricted to phone/portrait mode only for now. - [x] 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