Fixed thread safety for NetworkHelper.Instance (#3490)

## Fixes #3197
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<!-- Add a brief overview here of the feature/bug & fix. -->

## 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) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
<!-- - Sample app changes -->
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## What is the current behavior?
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->
The `NetworkHelper.Instance` property is not thread-safe.

## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->
The `NetworkHelper.Instance` property is now thread safe and initialized in the implicit static constructor.

## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] 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)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [ ] Contains **NO** breaking changes

Microsoft.Toolkit.Uwp.Connectivity/Network/NetworkHelper.cs

@@ -12,26 +12,11 @@ namespace Microsoft.Toolkit.Uwp.Connectivity
     /// </summary>
     public class NetworkHelper
     {
-        /// <summary>
-        /// Private singleton field.
-        /// </summary>
-        private static NetworkHelper _instance;
-
         /// <summary>
         /// Event raised when the network changes.
         /// </summary>
         public event EventHandler NetworkChanged;
 
-        /// <summary>
-        /// Gets public singleton property.
-        /// </summary>
-        public static NetworkHelper Instance => _instance ?? (_instance = new NetworkHelper());
-
-        /// <summary>
-        /// Gets instance of <see cref="ConnectionInformation"/>.
-        /// </summary>
-        public ConnectionInformation ConnectionInformation { get; } = new ConnectionInformation();
-
         /// <summary>
         /// Initializes a new instance of the <see cref="NetworkHelper"/> class.
         /// </summary>
@@ -52,6 +37,19 @@ protected NetworkHelper()
             NetworkInformation.NetworkStatusChanged -= OnNetworkStatusChanged;
         }
 
+        /// <summary>
+        /// Gets public singleton property.
+        /// </summary>
+        public static NetworkHelper Instance { get; } = new NetworkHelper();
+
+        /// <summary>
+        /// Gets instance of <see cref="ConnectionInformation"/>.
+        /// </summary>
+        public ConnectionInformation ConnectionInformation { get; }
+
+        /// <summary>
+        /// Checks the current connection information and raises <see cref="NetworkChanged"/> if needed.
+        /// </summary>
         private void UpdateConnectionInformation()
         {
             lock (ConnectionInformation)
@@ -69,6 +67,9 @@ private void UpdateConnectionInformation()
             }
         }
 
+        /// <summary>
+        /// Invokes <see cref="UpdateConnectionInformation"/> when the current network status changes.
+        /// </summary>
         private void OnNetworkStatusChanged(object sender)
         {
             UpdateConnectionInformation();