Allow overriding $(AndroidLatestApiLevel), $(AndroidLatestFrameworkVersion) (#640)

They lack Condition for emptiness check which prevents overriding.

Author: atsushieno

Configuration.props

@@ -14,8 +14,8 @@
   <PropertyGroup>
     <ProductVersion>7.4.99</ProductVersion>
     <!-- *Latest* API level binding that we support; used when building src/Xamarin.Android.Build.Tasks -->
-    <AndroidLatestApiLevel>25</AndroidLatestApiLevel>
-    <AndroidLatestFrameworkVersion>v7.1</AndroidLatestFrameworkVersion>
+    <AndroidLatestApiLevel Condition="'$(AndroidLatestApiLevel)' == ''">25</AndroidLatestApiLevel>
+    <AndroidLatestFrameworkVersion Condition="'$(AndroidLatestFrameworkVersion)' == ''">v7.1</AndroidLatestFrameworkVersion>
   </PropertyGroup>
   <PropertyGroup>
     <AutoProvision Condition=" '$(AutoProvision)' == '' ">False</AutoProvision>