Bump $(ProductVersion) to 8.1.99

(Commercial) Xamarin.Android v8.1 is being tracked in the
[xamarin-android/d15-5][0] branch, which was branched from
commit 100ca99b.

Bump `$(ProductVersion)` to 8.1.99 to track development progress of
the *next* version of Xamarin.Android.

[0]: https://github.com/xamarin/xamarin-android/commits/d15-5

Configuration.props

@@ -12,7 +12,7 @@
       Condition=" Exists('$(MSBuildThisFileDirectory)Configuration.OperatingSystem.props') And '$(DoNotLoadOSProperties)' != 'True' "
   />
   <PropertyGroup>
-    <ProductVersion>8.0.99</ProductVersion>
+    <ProductVersion>8.1.99</ProductVersion>
     <!-- *Latest* API level binding that we support; used when building src/Xamarin.Android.Build.Tasks -->
     <AndroidLatestApiLevel Condition="'$(AndroidLatestApiLevel)' == ''">26</AndroidLatestApiLevel>
     <AndroidLatestFrameworkVersion Condition="'$(AndroidLatestFrameworkVersion)' == ''">v8.0</AndroidLatestFrameworkVersion>