[Xamarin.Android.Build.Tasks] ignore .csproj.user file in $(MSBuildAllProjects)

[jonathanpeppers]

I noticed that changing the selected device or emulator in Visual
Studio 2019 16.8 on Windows was causing many MSBuild targets to run
again in the IDE. The time taken was similar to a Rebuild!

Looking closer, I noticed:

Building target "_CompileJava" completely.
Input file "App42.csproj.user" is newer than output file "obj\Debug\_javac.stamp".

.csproj.user is where the IDE stores the selected device or
emulator. The contents of the file:

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="Current" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <PropertyGroup>
    <SelectedDevice>pixel_2_pie_9_0_-_api_28</SelectedDevice>
    <DefaultDevice>pixel_2_pie_9_0_-_api_28</DefaultDevice>
  </PropertyGroup>
</Project>

The reason this triggers many targets is the file is actually imported:

https://github.com/dotnet/msbuild/blob/7452552ce911efdca6aea5a189c409f083db7bc7/src/Tasks/Microsoft.Common.CurrentVersion.targets#L31

And so this file is automatically included in $(MSBuildAllProjects),
which is used by many Inputs of Xamarin.Android MSBuild targets.

I first tried editing $(MSBuildAllProjects), but this property
appears to be readonly, since it is a built-in property in MSBuild.
Attempts at setting it were ignored.

To solve this, lets create a new @(_AndroidMSBuildAllProjects) item
group to be used instead that excludes the .csproj.user file.

I was able to reproduce this issue in a new test in
IncrementalBuildTest.

After this goes in, there is one place in the xamarin/monodroid repo
that needs to use @(_AndroidMSBuildAllProjects) instead of
$(MSBuildAllProjects).

[jonathanpeppers]

I manually tested a build from here, and everything seems to work switching from x86 emulator to x86_64 to arm64-v8a device.

I also edited the .csproj to make sure the targets that should run, still do.