[Mono.Android, Mono.Android.Export] Sign assemblies. (#29)

Sign the Mono.Android.dll, Mono.Android.Export.dll,
Xamarin.Android.NUniteLite.dll with the Xamarin.Android public key.

This will allow external contributors to seamlessly update their apps
with a custom Mono.Android.dll for bug fixes/etc. without errors [0]:

	@borgdylan:
		Mono.Android.dll does not have a public key...
		This causes compile errors when using teh support library v4.

[0]: https://gitter.im/xamarin/xamarin-android?at=572da219f36daf63798d6151

Author: jonpryor

external/Java.Interop

@@ -8,6 +8,8 @@
     <RootNamespace>Mono.Android.Export</RootNamespace>
     <AssemblyName>Mono.Android.Export</AssemblyName>
     <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
+    <SignAssembly>true</SignAssembly>
+    <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
   <Import Project="..\..\Configuration.props" />
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">

product.snk

@@ -8,6 +8,8 @@
     <RootNamespace>Android</RootNamespace>
     <AssemblyName>Mono.Android</AssemblyName>
     <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
+    <SignAssembly>true</SignAssembly>
+    <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
   <Import Project="..\..\Configuration.props" />
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">

src/Mono.Android.Export/Mono.Android.Export.csproj

@@ -11,6 +11,8 @@
     <MonoAndroidResourcePrefix>Resources</MonoAndroidResourcePrefix>
     <AssemblyName>Xamarin.Android.NUnitLite</AssemblyName>
     <AndroidResgenFile>Resources\Resource.designer.cs</AndroidResgenFile>
+    <SignAssembly>true</SignAssembly>
+    <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>
   </PropertyGroup>
   <Import Project="..\..\Configuration.props" />
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">