[video_player] Simplify Pigeon API on Android

[stuartmorgan]

The Pigeon API for video_player_android dates back to the earliest versions of Pigeon, when methods had to take only a single class parameter, and return a class. This updates the API surface to modern Pigeon style, eliminating wrapper classes (except for CreateMessage, which is complex enough that the class is useful).

Pre-launch Checklist

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