[Android] Add support for Android 14 Media Picker and partial photo access

[Copilot]

Overview

This PR enhances the MediaPicker implementation for Android 14 (API level 34) to support two key features:

1. Partial photo/video access using the new READ_MEDIA_VISUAL_USER_SELECTED permission
2. Improved bottom sheet UI for selecting photos and videos directly from the current page

Changes

1. Added READ_MEDIA_VISUAL_USER_SELECTED permission support

• Updated Permissions.Photos class to request the new permission on Android 14+
• Added conditional permission check in PickAsync method

2. Improved UI experience for Android 14

• Modified IntermediateActivity.StartAsync to detect Android 14+ and media picker requests
• For Android 14+, bypassed the intermediate activity to prevent showing an empty transition page
• Added direct activity result handling for media picker operations

3. Added developer integration helpers

• Created HandleMediaPickerResult extension method for processing media picker results
• Added documentation comments to explain the Android 14-specific handling

Developer Integration

App developers need to:

1. Add the READ_MEDIA_VISUAL_USER_SELECTED permission to their AndroidManifest.xml
2. Add the following code to their MainActivity:

protected override void OnActivityResult(int requestCode, Result resultCode, Intent? data)
{
    if (!this.HandleMediaPickerResult(requestCode, resultCode, data))
    {
        base.OnActivityResult(requestCode, resultCode, data);
    }
}

Fixes #21071.

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