Fixed UWP and Xamarin build (#574)

Fixed the failing Azure pipeline UWP build:
Changed generic EventHandler to now as PublishStateChangedEventHandler typed EventHandler. This fixed the errors in the UWP build.

Fixed the Xamarin build:
The msbuild for Android SDK 25 (Android 7.1) seems to be non-functional. Could not reproduce the exact same behavior locally - might also be caused by an error in the Azure pipeline configuration. (If the pipeline fails in the PR, I'll just remove the Xamarin related change...)

Samples/Controls/Subscriptions/SubscriptionDlg.xaml.cs

@@ -77,15 +77,15 @@ public SubscriptionDlg()
 
             m_SessionNotification = new NotificationEventHandler(Session_Notification);
             m_SubscriptionStateChanged = new SubscriptionStateChangedEventHandler(Subscription_StateChanged);
-            m_PublishStatusChanged = new EventHandler(Subscription_PublishStatusChanged);
+            m_PublishStatusChanged = new PublishStateChangedEventHandler(Subscription_PublishStatusChanged);
         }
         #endregion
 
         #region Private Fields
         private Subscription m_subscription;
         private NotificationEventHandler m_SessionNotification;
         private SubscriptionStateChangedEventHandler m_SubscriptionStateChanged;
-        private EventHandler m_PublishStatusChanged;
+        private PublishStateChangedEventHandler m_PublishStatusChanged;
         #endregion
 
         #region Public Interface

Samples/XamarinClient/XamarinClient.Android/UA Xamarin Client.Android.csproj

@@ -8,7 +8,7 @@
     <OutputType>Library</OutputType>
     <RootNamespace>XamarinClient.Droid</RootNamespace>
     <AssemblyName>XamarinClient.Android</AssemblyName>
-    <TargetFrameworkVersion>v7.1</TargetFrameworkVersion>
+    <TargetFrameworkVersion>v8.1</TargetFrameworkVersion>
     <AndroidApplication>True</AndroidApplication>
     <AndroidResgenFile>Resources\Resource.designer.cs</AndroidResgenFile>
     <AndroidResgenClass>Resource</AndroidResgenClass>
@@ -98,4 +98,4 @@
   </ItemGroup>
   <ItemGroup />
   <Import Project="$(MSBuildExtensionsPath)\Xamarin\Android\Xamarin.Android.CSharp.targets" />
-</Project>
+</Project>