Support Mice in iPadOS 13.4+

[gw280]

Description

Adds support for iOS 13.4 mouse support.

Related Issues

Prior PR is #19965
flutter/flutter#54663
flutter/flutter#55809

Tests

Tests TBA.

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Breaking Change

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[flutter-dashboard]

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[gaaclarke]

Code looks good, just a few nits. What are your plans for testing this?

[gaaclarke]

lgtm, nice