Add BackgroundTaskManager and WaitForBackgroundTasks

[dellis1972]

Add support classes to allow uses to kick off a background task and then wait for it to complete later in the build process.

[jonathanpeppers]

Does this repo have a test project? We need to add some?

[dellis1972]

So after our chat the other week I am proposing we now do the following.
Expose two properties on the AsyncTask.

public string Category { get; set;} = "default";
public bool BackgroundTask { get; set;} = false;

by default the AsyncTask will work as it does now. However if you set BackgroundTask = true you still need to call base.Exexute but instead of waiting there and then it will register the actual AsyncTask with the background manager.

You can then use the WaitForBackgroundTasks later on in the process as we do now.
I think this makes it less complicated for the end user. They just set a property and call WaitForBackgroundTasks and are done.

[dellis1972]

/azp build

[azure-pipelines]

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[dellis1972]

/azp run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 6 in repo xamarin/Xamarin.Build.AsyncTask

[jonathanpeppers]

Why does it tell you how to run commands then say you aren't allowed???

/azp lol

[dellis1972]

/azp run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 6 in repo xamarin/Xamarin.Build.AsyncTask

[jonathanpeppers]

Does this method need the Yield, try-finally, Reacquire calls like we do here?

https://github.com/xamarin/xamarin-android/blob/fd64aaff618f5f818831bdef844876e138fec7a0/src/Xamarin.Android.Build.Tasks/Tasks/AndroidTask.cs#L47-L60