Document Animation callbacks

[jonathanpeppers]

Summary

• Documents Animation lifecycle callbacks plus values for animation duration, background color, and wallpaper detachment.

Refs #244

Sources

• Official Android reference: https://developer.android.com/reference/android/view/animation/Animation
• AOSP Animation.java (callback, duration, and property documentation): https://android.googlesource.com/platform/frameworks/base/+/refs/heads/main/core/java/android/view/animation/Animation.java

Validation

• XML parsed successfully with PowerShell [xml].
• Confirmed CRLF-only line endings and a trailing CRLF.
• Reduced Android.Views.AnimationsTo be added placeholders from 110 to 104; Animation.xml retains only the two .NET infrastructure JniPeerMembers placeholders.
• git diff --check passed.

[Copilot]

Pull request overview

This PR improves the API reference XML documentation for Android.Views.Animations.Animation by replacing several remaining placeholder docs with meaningful descriptions, focusing on animation lifecycle callbacks and a few related properties/methods.

Changes:

• Documented AnimationStart, AnimationRepeat, and AnimationEnd event summaries.
• Replaced placeholder <value> documentation for BackgroundColor and DetachWallpaper.
• Replaced placeholder <returns> documentation for ComputeDurationHint().

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[jonathanpeppers]

@dalexsoto review

[dalexsoto]

AnimationEnd says the event is never raised for infinite-repeat animations, but a started infinite animation does raise it when Animation.Cancel() invokes fireAnimationEnd(). Please qualify the statement as applying to natural completion or explicitly document cancellation.