Add missing media3 Stub OnPositionDiscontinuity back to Android Med…

[ne0rrmatrix]

Description of Change

Add missing media3 Stub OnPositionDiscontinuity back to Android MediaManager class

PR Classification

New feature to enhance media playback handling.

PR Summary

This pull request introduces a new method to improve the media manager's ability to respond to playback position changes.

• MediaManager.android.cs: Added the OnPositionDiscontinuity method with parameters for old and new positions and a reason.

Linked Issues

• Fixes [BUG] MediaElement: New "onPositionDiscontinuity" Method not implemented for new Android ExoPlayer refactor, causing crashes on mediaElement.Stop() - stub needs adding to MediaManager #2824

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

This is the last Stub I had forgotten to add back. There was a specific reason why I removed it. dotnet/android-libraries#929 & dotnet/android-libraries#949

There are issues with having it and not having this specific stub in media element. I am not sure exactly what issues it will generate by adding it back now. It may cause no problem for developers. But I do remember that it was excluded for a reason that caused actual issues for some people. I don't remember what issues as it was a while back.