[Java.Interop] Allow $(TargetFrameworkVersion) to be overridden (#364)

Context: http://work.devdiv.io/667174
Context: dotnet/android#2105
Context: 893562c

Turns Out™ that resolving PCL assemblies within Xamarin.Android is
not as fast as resolving `MonoAndroid`-profile assemblies; see commit
893562c, wherein we see that non-PCL use is 30-50% faster than PCL
assembly use -- shaving off ~4 seconds -- for unknown reasons.

Run with it: allow `Java.Interop.dll` to be built as a
`MonoAndroid,v1.0`-profile assembly in addition to a PCL.
To do this we need to allow `$(TargetFrameworkVersion)` to be
overridden "elsewhere" -- specifically in `Mono.Android.targets`
within the xamarin-android repo -- otherwise `Java.Interop.dll`
will be built for the *wrong* framework profile.

Author: jonathanpeppers

src/Java.Interop/Java.Interop.csproj

@@ -10,7 +10,7 @@
     <OutputType>Library</OutputType>
     <RootNamespace>Java.Interop</RootNamespace>
     <AssemblyName>Java.Interop</AssemblyName>
-    <TargetFrameworkVersion>v4.5</TargetFrameworkVersion>
+    <TargetFrameworkVersion Condition=" '$(TargetFrameworkVersion)' == '' ">v4.5</TargetFrameworkVersion>
     <NoWarn>1591</NoWarn>
     <SignAssembly>true</SignAssembly>
     <AssemblyOriginatorKeyFile>..\..\product.snk</AssemblyOriginatorKeyFile>