[Xamarin.Android.Build.Tasks] Set TargetFrameworkRootPath in props (dotnet#573)

Always set `$(TargetFrameworkRootPath)` in props as well as
`$(FrameworkPathOverride)`, as this allows users to override the
value if they choose to.

Also, if `$(TargetFrameworkRootPath)` isn't set by us, it's not set
in the project evaluation phase, so by the time we set
`$(FrameworkPathOverride)` it's still empty and wrong for
Visual Studio 2013/2015.

The newly added `Xamarin.Android.Sdk.targets` also allows us to add
the global targets override `Xamarin.Sdk.targets` to be consistent
with the `Xamarin.Sdk.props` file that we already import. This allows
IDEs to provide additional behavior as needed that is cross-cutting
across all Xamarin builds.

Author: kzu

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Bindings.After.targets

@@ -0,0 +1,20 @@
+<!--
+***********************************************************************************************
+Xamarin.Android.Bindings.Before.targets
+
+WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and have
+  created a backup copy.  Incorrect changes to this file will make it
+  impossible to load or build your projects from the command-line or the IDE.
+
+Copyright (C) 2014 Xamarin. All rights reserved.
+***********************************************************************************************
+-->
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <!-- Allows providing SDK-specific targets overrides -->
+  <PropertyGroup>
+    <XamarinAndroidSdkTargets>$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), 'Xamarin.Android.Sdk.targets'))\Xamarin.Android.Sdk.targets</XamarinAndroidSdkTargets>
+  </PropertyGroup>
+  <Import Project="$(XamarinAndroidSdkTargets)" Condition="'$(XamarinAndroidSdkTargetsImported)' != 'true' And Exists('$(XamarinAndroidSdkTargets)')"/>
+
+</Project>

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Android/Xamarin.Android.Common.After.targets

@@ -14,4 +14,10 @@ Copyright (C) 2014 Xamarin. All rights reserved.
     <Import Project="$(MSBuildThisFileDirectory)\Xamarin.Android.Common\ImportAfter\*"
             Condition="Exists('$(MSBuildThisFileDirectory)\Xamarin.Android.Common\ImportAfter')"/>
 
+    <!-- Allows providing SDK-specific targets overrides -->
+    <PropertyGroup>
+        <XamarinAndroidSdkTargets>$([MSBuild]::GetDirectoryNameOfFileAbove($(MSBuildThisFileDirectory), 'Xamarin.Android.Sdk.targets'))\Xamarin.Android.Sdk.targets</XamarinAndroidSdkTargets>
+    </PropertyGroup>
+    <Import Project="$(XamarinAndroidSdkTargets)" Condition="'$(XamarinAndroidSdkTargetsImported)' != 'true' And Exists('$(XamarinAndroidSdkTargets)')"/>
+
 </Project>

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Xamarin.Android.Sdk.props

@@ -19,12 +19,14 @@ Copyright (C) 2011-2016 Xamarin. All rights reserved.
 		<VsInstallationID>$(DevEnvIni.Substring($(InstallationIDEqualsIndex), 8))</VsInstallationID>
 	</PropertyGroup>
 
-	<PropertyGroup Condition="'$(VsInstallRoot)' != ''">
+	<PropertyGroup>
 		<!-- Until VS2017+ includes its own ReferenceAssemblies outside of C:\Program Files (x86)\Reference Assemblies and into 
 			 the VsInstallRoot, we must override this ourselves for our SDKs -->
-		<TargetFrameworkRootPath Condition="'$(TargetFrameworkRootPath)' == ''">$(VsInstallRoot)\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\</TargetFrameworkRootPath>
+		<TargetFrameworkRootPath Condition="'$(VsInstallRoot)' != '' And '$(TargetFrameworkRootPath)' == ''">$(VsInstallRoot)\Common7\IDE\ReferenceAssemblies\Microsoft\Framework\</TargetFrameworkRootPath>
+		<TargetFrameworkRootPath Condition="'$(VsInstallRoot)' == '' And '$(TargetFrameworkRootPath)' == ''">$(ProgramFiles)\Reference Assemblies\Microsoft\Framework\</TargetFrameworkRootPath>
+
+		<!-- We should always override the framework path so that XA resolves to its own mscorlib.dll -->
 		<FrameworkPathOverride>$(TargetFrameworkRootPath)MonoAndroid\v1.0</FrameworkPathOverride>
-		<XamarinAndroidSdkPropsImported>true</XamarinAndroidSdkPropsImported>
 	</PropertyGroup>
 
 	<Target Name="RedirectMonoAndroidSdkPaths" Condition="'$(VsInstallRoot)' != ''">
@@ -53,6 +55,10 @@ Copyright (C) 2011-2016 Xamarin. All rights reserved.
 		</Task>
 	</UsingTask>
 
+	<PropertyGroup>
+		<XamarinAndroidSdkPropsImported>true</XamarinAndroidSdkPropsImported>
+	</PropertyGroup>
+
 	<!-- Allows providing Xamarin SDK-specific property overrides -->
 	<Import Project="Xamarin.Sdk.props" Condition="'$(XamarinSdkPropsImported)' != 'true' And Exists('Xamarin.Sdk.props')"/>
-</Project>
+</Project>

src/Xamarin.Android.Build.Tasks/MSBuild/Xamarin/Xamarin.Android.Sdk.targets

@@ -0,0 +1,20 @@
+<!--
+***********************************************************************************************
+Xamarin.Android.Sdk.targets
+
+WARNING:  DO NOT MODIFY this file unless you are knowledgeable about MSBuild and have
+  created a backup copy.  Incorrect changes to this file will make it
+  impossible to load or build your projects from the command-line or the IDE.
+
+Copyright (C) 2011-2016 Xamarin. All rights reserved.
+***********************************************************************************************
+-->
+<Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+
+  <PropertyGroup>
+    <XamarinAndroidSdkTargetsImported>true</XamarinAndroidSdkTargetsImported>
+  </PropertyGroup>
+
+  <!-- Allows providing Xamarin SDK-specific targets overrides -->
+	<Import Project="Xamarin.Sdk.targets" Condition="'$(XamarinSdkTargetsImported)' != 'true' And Exists('Xamarin.Sdk.targets')"/>
+</Project>