[BaseTasks] Import Xamarin.Build.AsyncTask (#237)

We've decided to move https://github.com/xamarin/Xamarin.Build.AsyncTask
into this repository as part of dotnet org migration efforts.

Source and test content from xamarin/Xamarin.Build.AsyncTask@db4ce14dac
has been added to Microsoft.Android.Build.BaseTasks.

The `AndroidAsyncTask` and `AsyncTask` classes have been merged,
we've decided we no longer need both levels of abstraction.

`Microsoft.Android.Build.BaseTasks-Tests` has been updated to target
net8.0 as I noticed the tests were no longer running:

    /Users/runner/hostedtoolcache/dotnet/dotnet test /Users/runner/work/1/s/bin/TestDebug-net6.0/Microsoft.Android.Build.BaseTasks-Tests.dll --logger trx --results-directory /Users/runner/work/_temp
    Starting test execution, please wait...
    A total of 1 test files matched the specified pattern.
    No test is available in /Users/runner/work/1/s/bin/TestDebug-net6.0/Microsoft.Android.Build.BaseTasks-Tests.dll. Make sure that test discoverer & executors are registered and platform & framework version settings are appropriate and try again.

Author: pjcollins

azure-pipelines.yaml

@@ -41,6 +41,11 @@ jobs:
     inputs:
       version: $(DotNetCoreVersion)
 
+  - task: UseDotNet@2
+    displayName: Use .NET Core 8.0.x
+    inputs:
+      version: 8.0.x
+
   - task: DotNetCoreCLI@2
     displayName: Build solution Xamarin.Android.Tools.sln
     inputs: