[Xamarin.Forms-Performance-Integration] Use AndroidFrameworkVersion (#1004)

Other Android projects (such as test APKs), set `$(TargetFrameworkVersion)`
by importing `Configuration.props` and using `$(AndroidFrameworkVersion)`.
`$(AndroidUseLatestPlatformSdk)` should also be set to `false`.

Not sure why this hasn't caused any issues yet, I first noticed this
causing a failure on PR #997.

tests/Xamarin.Forms-Performance-Integration/Droid/Xamarin.Forms.Performance.Integration.Droid.csproj

@@ -8,14 +8,17 @@
     <OutputType>Library</OutputType>
     <RootNamespace>Xamarin.Forms.Performance.Integration.Droid</RootNamespace>
     <AssemblyName>Xamarin.Forms.Performance.Integration.Droid</AssemblyName>
-    <TargetFrameworkVersion>v7.1</TargetFrameworkVersion>
     <AndroidApplication>True</AndroidApplication>
     <AndroidResgenFile>Resources\Resource.designer.cs</AndroidResgenFile>
     <AndroidResgenClass>Resource</AndroidResgenClass>
     <AndroidManifest>Properties\AndroidManifest.xml</AndroidManifest>
     <MonoAndroidResourcePrefix>Resources</MonoAndroidResourcePrefix>
     <MonoAndroidAssetsPrefix>Assets</MonoAndroidAssetsPrefix>
-    <AndroidUseLatestPlatformSdk>true</AndroidUseLatestPlatformSdk>
+    <AndroidUseLatestPlatformSdk>false</AndroidUseLatestPlatformSdk>
+  </PropertyGroup>
+  <Import Project="..\..\..\Configuration.props" />
+  <PropertyGroup>
+    <TargetFrameworkVersion>$(AndroidFrameworkVersion)</TargetFrameworkVersion>
   </PropertyGroup>
   <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
     <DebugSymbols>true</DebugSymbols>