Draft: Remove duplicate code from player view controllers

PlayerViewController and VideoPlayerViewController had some duplicate methods. This MR attempts to merge them appropriately.

PlayerViewController keeps brightness code since most of it lived there anyway, and it's conceivable that a future subclass might want that functionality.

VideoPlayerViewController had two implementations of the same method, which I did not know was possible. I wonder if it is a compiler bug.

Most of this is from !1440 but done more completely and thoughtfully.

Marked as draft due to lack of on-device testing.