Update video_player_platform_interface to latest pigeon (flutter#3507)

See flutter/plugins#3281 for context.

Author: Hixie

packages/video_player/video_player_platform_interface/CHANGELOG.md

@@ -1,3 +1,8 @@
+## 4.0.0-nullsafety.0
+
+* Update to latest Pigeon.
+  This includes a breaking change to how the test logic is exposed.
+
 ## 3.0.0-nullsafety.3
 
 * `messages.dart` sets Dart `2.12`.