[Microsoft.Android.Sdk] use net5.0-android $(TargetFramework)

After bumping to .NET 5.0.100-rc.1.20411.10, we can now use:

    <TargetFramework>net5.0-android</TargetFramework>

On the first try, this failed with:

    Microsoft.NET.TargetFrameworkInference.targets(113,5): error NETSDK1139: The target platform identifier android was not recognized.

Our SDK needs to specify:

    <TargetPlatformSupported Condition=" '$(TargetPlatformIdentifier)' == 'Android' ">true</TargetPlatformSupported>

After doing this, everything seemed to work as before.

I updated tests so they will use `net5.0-android` going forward.

Author: jonathanpeppers

Documentation/guides/OneDotNet.md

@@ -206,7 +206,7 @@ The following instructions can be used for early preview testing.
 ```xml
 <Project Sdk="Microsoft.Android.Sdk/10.0.100">
   <PropertyGroup>
-    <TargetFramework>net5.0</TargetFramework>
+    <TargetFramework>net5.0-android</TargetFramework>
     <RuntimeIdentifier>android.21-arm64</RuntimeIdentifier>
     <OutputType>Exe</OutputType>
   </PropertyGroup>

build-tools/automation/azure-pipelines.yaml

@@ -52,7 +52,7 @@ variables:
   NUnitConsoleVersion: 3.11.1
   DotNetCoreVersion: 3.1.201
   # Version number from: https://github.com/dotnet/installer#installers-and-binaries
-  DotNetCorePreviewVersion: 5.0.100-preview.7.20307.3
+  DotNetCorePreviewVersion: 5.0.100-rc.1.20411.10
   HostedMacMojave: Hosted Mac Internal Mojave
   HostedMac: Hosted Mac Internal
   HostedWinVS2019: Hosted Windows 2019 with VS2019

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

@@ -1,6 +1,7 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 
   <PropertyGroup>
+    <TargetPlatformSupported Condition=" '$(TargetPlatformIdentifier)' == 'Android' ">true</TargetPlatformSupported>
     <MicrosoftAndroidSdkTargetsImported>true</MicrosoftAndroidSdkTargetsImported>
     <_XamarinAndroidBuildTasksAssembly>$(MSBuildThisFileDirectory)..\tools\Xamarin.Android.Build.Tasks.dll</_XamarinAndroidBuildTasksAssembly>
   </PropertyGroup>

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Android/XASdkProject.cs

@@ -48,7 +48,7 @@ static XASdkProject ()
 		public XASdkProject (string outputType = "Exe")
 		{
 			Sdk = $"Microsoft.Android.Sdk/{SdkVersion}";
-			TargetFramework = "net5.0";
+			TargetFramework = "net5.0-android";
 
 			TargetSdkVersion = AndroidSdkResolver.GetMaxInstalledPlatform ().ToString ();
 			PackageName = PackageName ?? string.Format ("{0}.{0}", ProjectName);

src/Xamarin.Android.Build.Tasks/Tests/Xamarin.ProjectTools/Common/DotNetXamarinProject.cs

@@ -26,7 +26,7 @@ protected DotNetXamarinProject (string debugConfigurationName = "Debug", string
 			SetProperty ("AssemblyName", () => AssemblyName ?? ProjectName);
 
 			if (Builder.UseDotNet) {
-				SetProperty ("TargetFramework", "net5.0");
+				SetProperty ("TargetFramework", "net5.0-android");
 				SetProperty ("EnableDefaultItems", "false");
 				SetProperty ("AppendTargetFrameworkToOutputPath", "false");
 				SetProperty ("AppendRuntimeIdentifierToOutputPath", "false");