title: Can't build Xamarin Android project in Azure Pipelines
body: <p>I'm trying to create a basic build pipeline in Azure DevOps which builds a Visual Studio solution that includes .NET Core / .NET Standard projects and a Xamarin.Android project. The solution builds locally in VS 2019 with no issues, but always fails on the build agent with these build errors:</p>
  
  <pre><code>Error APT2260: resource style/Theme.AppCompat.Light.Dialog (aka com.companyname.obrien.connect.forms:style/Theme.AppCompat.Light.Dialog) not found.
  
  Source\Obrien.Connect.Forms.Android\Resources\values\styles.xml(4,0): Error APT2260: style attribute 'attr/colorAccent (aka com.companyname.obrien.connect.forms:attr/colorAccent)' not found.
  
  Error APT2260: resource style/Theme.AppCompat.Light.DarkActionBar (aka com.companyname.obrien.connect.forms:style/Theme.AppCompat.Light.DarkActionBar) not found.
  
  Source\Obrien.Connect.Forms.Android\Resources\values\styles.xml(2,0): Error APT2260: style attribute 'attr/windowNoTitle (aka com.companyname.obrien.connect.forms:attr/windowNoTitle)' not found.
  
  Source\Obrien.Connect.Forms.Android\Resources\values\styles.xml(2,0): Error APT2260: style attribute 'attr/windowActionBar (aka com.companyname.obrien.connect.forms:attr/windowActionBar)' not found.
  
  Source\Obrien.Connect.Forms.Android\Resources\values\styles.xml(2,0): Error APT2260: style attribute 'attr/colorPrimary (aka com.companyname.obrien.connect.forms:attr/colorPrimary)' not found.
  
  Source\Obrien.Connect.Forms.Android\Resources\values\styles.xml(2,0): Error APT2260: style attribute 'attr/colorPrimaryDark (aka com.companyname.obrien.connect.forms:attr/colorPrimaryDark)' not found.
  
  Source\Obrien.Connect.Forms.Android\Resources\values\styles.xml(3,0): Error APT2260: style attribute 'attr/colorAccent (aka com.companyname.obrien.connect.forms:attr/colorAccent)' not found.
  
  Source\Obrien.Connect.Forms.Android\Resources\values\styles.xml(4,0): Error APT2260: style attribute 'attr/windowActionModeOverlay (aka com.companyname.obrien.connect.forms:attr/windowActionModeOverlay)' not found.
  
  C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\MSBuild\Xamarin\Android\Xamarin.Android.Aapt2.targets(155,3): Error APT2260: resource style/TextAppearance.AppCompat.Button (aka com.companyname.obrien.connect.forms:style/TextAppearance.AppCompat.Button) not foun
  </code></pre>
  
  <p>This is the YAML for the pipeline:</p>
  
  <pre><code>trigger:
  - develop
  - feature/*
  
  pool:
    vmImage: 'windows-2019'
  
  variables:
  - group: 'ci-build'
  
  steps:
  - task: NuGetToolInstaller@1
    displayName: 'Install NuGet 5.4.0' 
    inputs:
      versionSpec: '5.4.0'
  
  - task: DotNetCoreCLI@2
    displayName: 'Restore .NET Packages'
    inputs:
      command: restore
      projects: '**/OBrien.Connect.Forms*/*.csproj'
      verbosityRestore: minimal
  
  - task: NuGetCommand@2
    displayName: 'Restore Android Packages'
    inputs:
      command: 'restore'
      restoreSolution: '**/OBrien.Connect.Forms.sln'
  
  - task: VSBuild@1
    displayName: 'Build Solution'
    inputs:    
      solution: '**/$(solutionName)' 
      vsVersion: '16.0'
      configuration: '$(buildConfiguration)'
  </code></pre>
  
  <p>I needed to use <code>dotnet restore</code> on the projects in the solution first, so that I could build them in the subsequent VSBuild task, that works fine. However, this doesn't restore any packages needed by the Xamarin.Android project as that is based on Mono and is ignored by the first restore.</p>
  
  <p>That's why I added the second NuGet restore on the entire solution, but this never does anything - no errors, just this output:</p>
  
  <pre><code>##[section]Starting: Restore Android Packages
  ==============================================================================
  Task         : NuGet
  Description  : Restore, pack, or push NuGet packages, or run a NuGet command. Supports NuGet.org and authenticated feeds like Azure Artifacts and MyGet. Uses NuGet.exe and works with .NET Framework apps. For .NET Core and .NET Standard apps, use the .NET Core task.
  Version      : 2.161.1
  Author       : Microsoft Corporation
  Help         : https://docs.microsoft.com/azure/devops/pipelines/tasks/package/nuget
  ==============================================================================
  SYSTEMVSSCONNECTION exists true
  SYSTEMVSSCONNECTION exists true
  [command]C:\windows\system32\chcp.com 65001
  Active code page: 65001
  Detected NuGet version 5.4.0.6315 / 5.4.0+d790b66be476cd901a56bd46ada037162097ee21.d790b66be476cd901a56bd46ada037162097ee21
  SYSTEMVSSCONNECTION exists true
  Saving NuGet.config to a temporary config file.
  [command]C:\hostedtoolcache\windows\NuGet\5.4.0\x64\nuget.exe sources Add -NonInteractive -Name NuGetOrg -Source https://api.nuget.org/v3/index.json -ConfigFile D:\a\1\Nuget\tempNuGet_552.config
  Package source with Name: NuGetOrg added successfully.
  ##[section]Finishing: Restore Android Packages
  </code></pre>
  
  <p>I've tried using the XamarinAndroid@1 build task instead of building the whole solution, but it has exactly the same build errors.</p>