Create separate instances of GazePointer per (UI) thread, (#3590)

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## Fixes #
Fixes #3589 
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## PR Type
What kind of change does this PR introduce?
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<- Bugfix 
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
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## What is the current behavior?
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One GazePointer object per application.

## What is the new behavior?
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One GazePointer object per (UI) thread.

## PR Checklist

Please check if your PR fulfills the following requirements:

- [ ] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->
- [ ] Sample in sample app has been added / updated (for bug fixes / features)
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- [X] Contains **NO** breaking changes

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## Other information

Author: msftbot[bot]

Microsoft.Toolkit.Uwp.Input.GazeInteraction/GazePointer.cs

@@ -6,6 +6,7 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Threading;
 using System.Threading.Tasks;
 using Windows.Devices.Input.Preview;
 using Windows.Foundation;
@@ -298,20 +299,9 @@ internal int CursorRadius
         /// </summary>
         public bool IsAlwaysActivated { get; set; }
 
-        private static GazePointer _instance = null;
+        private static ThreadLocal<GazePointer> _instance = new ThreadLocal<GazePointer>(() => new GazePointer());
 
-        internal static GazePointer Instance
-        {
-            get
-            {
-                if (_instance == null)
-                {
-                    _instance = new GazePointer();
-                }
-
-                return _instance;
-            }
-        }
+        internal static GazePointer Instance => _instance.Value;
 
         internal void AddRoot(int proxyId)
         {