[video_player] Improve seek performance on Android

[sailendrabathi]

Adds backBufferDurationMs to VideoPlayerOptions and configures ExoPlayer's DefaultLoadControl to retain the back buffer from keyframes. This significantly improves seek performance and responsiveness during video playback on Android.

iOS already has some kind of back buffering implemented in AVPlayer because the replays and seeks are instantaneous for small videos.

Before:
Replays and seeks on Android for network based video assets involve a network call to fetch the video content. This significantly hampers the user experience.

After:
Replays and seeks on Android (within the back buffer duration) are instant just like in iOS. Improving user experience

This PR partly addresses flutter/flutter#97428. Not enabling back buffering by default and instead allowing the app to configure as per need. The default behavior will be same as the existing implementation to avoid any regressions.

Pre-Review Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [AI contribution guidelines] and understand my responsibilities, or I am not using AI tools.
• I read the [Tree Hygiene] page, which explains my responsibilities.
• I read and followed the [relevant style guides] and ran [the auto-formatter].
• I signed the [CLA].
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I [linked to at least one issue that this PR fixes] in the description above.
• I followed [the version and CHANGELOG instructions], using [semantic versioning] and the [repository CHANGELOG style], or I have commented below to indicate which documented exception this PR falls under[^1].
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which [test exemption] this PR falls under[^1].
• All existing and new tests are passing.

[gemini-code-assist]

Code Review

This pull request adds the backBufferDurationMs option to VideoPlayerOptions to configure the ExoPlayer back buffer duration on Android, updating the platform interface, Android implementation, and adding no-op stubs for AVFoundation and Web. Feedback on the changes highlights that passing backBufferDurationMs via a global setter on Android can cause state persistence and race conditions during concurrent player initialization, and recommends passing this option directly through CreationOptions instead.

[gemini-code-assist]

Code Review

This pull request adds support for configuring ExoPlayer's back buffer duration on Android by introducing backBufferDurationMs to VideoPlayerOptions and forwarding it through the platform interface to the Android implementation. The Android plugin updates PlatformViewVideoPlayer and TextureVideoPlayer to configure ExoPlayer's DefaultLoadControl using this value, supported by new unit tests. Feedback suggests correcting an inaccurate comment regarding how backBufferDurationMs is passed and adding an assertion to ensure the duration is positive if provided.