Feature request: Ability to hide onscreen controls programmatically

When working with PIP (picture-in-picture) mode:
Hiding player controls before transitioning to PIP mode is a must for user experience.
Something like ```controller.hideControls()``` would be great

Current behavior:
1. Go fullscreen
2. Tap to show player controls
3. Enter PIP mode
4. Player controls are still visible in the small PIP window (for a duration of _hideTimer which is 3s)

Suggested behavior:
1. Go fullscreen
2. Tap to show player controls
3. Call ```controller.hideControls()```
4. Enter PIP mode
4. Only video is showing in the small PIP window

I did implement this in my fork and it works. But I'm pretty new to flutter development and it kinda felt like a hack?
Here's the commit: https://github.com/kono0514/chewie_extended/commit/90b803d71f2ad308f03affd3314abbb188a859b9
If you can provide feedback for this commit, I can improve my implementation and do a PR.

Start by reviewing the controller API and the linked fork commit implementing hideControls(), then trace how controls are shown during fullscreen and picture-in-picture transitions. Done means exposing a supported programmatic way to hide the controls so the PIP window shows only video; no specific test or repository file is named.

Written by the indexing model from the issue text.