[video_player] Move more Java logic to Dart

[stuartmorgan-g]

Moves some logic from native code to Dart:

• User-Agent header -> ExoPlayer user agent extraction logic (with TODOs for future improvements, based on implementation issues/questions I noticed while converting it).
• Asset -> URL lookup.
• Conversion of buffer position to the buffer range list required for the Dart event stream.

Also converts video format hint from strings to enums, which was missed in earlier Pigeon conversions. Using strings was the pre-Pigeon way of handling this kind of thing.

Part of flutter/flutter#172763

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Code Review

The pull request moves video player logic from native code to Dart, improving maintainability. It also introduces an enum for video format hints. The review identifies a critical issue with a missing break statement in a switch block and a medium-severity issue with case-sensitive HTTP header lookup. Addressing these will improve the correctness and robustness of the code.