[Xamarin.Android.Build.Tasks] fix potential AsyncTask.Log calls

[jonathanpeppers]

Backport of: #4187

We have had a continuing concern of accidentally calling methods on
AsyncTask.Log from a background thread. If it happens, this can
cause a hang inside Visual Studio on Windows.

I finally had the idea of creating a build error if we inadvertently
do this:

[Obsolete("You should not use the 'Log' property directly for AsyncTask. Use the 'Log*' methods instead.", error: true)]
public new TaskLoggingHelper Log { get; }

This uncovered 26 build errors! Some were actually on the main
thread and completely OK, but several were not OK.

In most of the places, I could simply change Log.LogDebugMessage()
to LogDebugMessage().

However a few of the more troublesome places:

• ManifestDocument required a TaskLoggingHelper for its ctor.
  I moved this to only the methods that need it.

• NdkUtils and NdkUtilsOld I used Action<string> and
  Action<string, string> callbacks instead.

• The <Aot/> task had an empty ValidateAotConfiguration()
  method I removed.

Going forward, it should be a lot harder for us to introduce a hang
by using AsyncTask.Log.