[BaseTasks] Import Xamarin.Build.AsyncTask

[pjcollins]

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,
as we don't 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.

[pjcollins]

Other testing PRs:

https://github.com/xamarin/androidtools/pull/443
https://github.com/xamarin/android-sdk-installer/pull/966
https://github.com/xamarin/monodroid/pull/1497
dotnet/android#9017

[pjcollins]

@dellis1972@jonathanpeppers I think this should be ready for further review and hopefully good to go now. I just bumped Microsoft.Android.Build.BaseTasks-Tests to .NET 8 as I noticed those tests weren't actually running in main:
https://devdiv.visualstudio.com/DevDiv/_build/results?buildId=9730201&view=logs&j=3b059b26-55fe-5273-567a-5b3fa6c051e7&t=73552669-1d50-5779-0733-0594204b4f2d&l=16

/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.

The downstream PRs are all looking good so far against these latest changes as well.

[jonathanpeppers]

LGTM 👍 The main fallout will be bumping a lot of things everywhere!