add abstract base class for simple usage

Author: tmijieux

OneSignal.Android.Binding/Additions/NotificationExtenderBase.cs

@@ -0,0 +1,9 @@
+using AndroidX.Core.App;
+
+namespace Com.OneSignal.Android
+{
+    public abstract class NotificationExtenderBase : Java.Lang.Object, NotificationCompat.IExtender
+    {
+        public abstract NotificationCompat.Builder Extend(NotificationCompat.Builder builder);
+    }
+}

OneSignal.Android.Binding/Additions/OSRemoteNotificationReceivedBase.cs

@@ -0,0 +1,9 @@
+using Android.Content;
+
+namespace Com.OneSignal.Android
+{
+    public abstract class OSRemoteNotificationReceivedBase : Java.Lang.Object, OneSignal.IOSRemoteNotificationReceivedHandler
+    {
+        public abstract void RemoteNotificationReceived(Context ctx, OSNotificationReceivedEvent ev);
+    }
+}

OneSignal.Android.Binding/OneSignal.Android.Binding.csproj

@@ -43,6 +43,9 @@
     <Reference Include="System.Core" />
     <Reference Include="Mono.Android" />
   </ItemGroup>
+  <ItemGroup>
+    <Compile Include="Additions\*.cs"/>
+  </ItemGroup>
   <ItemGroup>
     <None Include="Additions\AboutAdditions.txt" />
     <None Include="Jars\AboutJars.txt" />