bump TargetFramework to MonoAndroid81

LibVLCSharp/LibVLCSharp.csproj

-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
   <PropertyGroup>
     <Title>LibVLCSharp</Title>
     <Summary>.NET bindings for LibVLC</Summary>
     <Description>.NET bindings for LibVLC</Description>
     <PackageTags>libvlc native xamarin .net video audio media mediaplayer</PackageTags>
-    <TargetFrameworks>netstandard2.0;MonoAndroid80;net461;netcoreapp2.0;Xamarin.iOS10</TargetFrameworks>
+    <TargetFrameworks>netstandard2.0;MonoAndroid81;net461;netcoreapp2.0;Xamarin.iOS10</TargetFrameworks>
     <RootNamespace>LibVLCSharp</RootNamespace>
     <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
     <DefineConstants>$(DefineConstants);</DefineConstants>