Update Xamarin.Forms version to 4.6.0-pre4

[rookiejava]

Description of Change

This PR is to update the Xamarin.Forms version to 4.6.0-pre4. As a result, unnecessary classes (MediaSource, EmbeddingControl, and so on) have been removed.

API Changes

Below removed classes/interfaces are replaced with the classes/interfaces added in Xamarin.Forms MediaElement PR.

Removed:

• public class MediaSourceHandler
• public class EmbeddingControls
• public class FileMediaSource
• public class FIleMediaSourceConverter
• public interface IPlatformMediaPlayer
• public interface IVideoOutput
• public class MediaSource
• public class MediaSourceConverter
• public class UriMediaSource

Below temporarily added classes will be removed when the related upstream Xamarin.Forms PR is merged,

namespace Tizen.Wearable.CircularUI.Forms

• interface IPlatformMediaPlayer2

namespace Tizen.Wearable.CircularUI.Forms.Renderer

• class MediaPlayerImpl
• class EmbeddingControls2

Behavioral Changes

You don't need to modify anything for this.