[Xamarin.Android.Build.Tasks] helpers for AsyncTask + TPL

[jonathanpeppers]

Unhandled exceptions from a Parallel.ForEach log an exception in
MSBuild such as:

E:\A\_work\1214\s\bin\Release\lib\xamarin.android\xbuild\Xamarin\Android\Xamarin.Android.Aapt2.targets(84,3): One or more errors occurred.
    at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean includeTaskCanceledExceptions)
    at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout, CancellationToken cancellationToken)
    at System.Threading.Tasks.Task.Wait()
    at System.Threading.Tasks.Parallel.ForWorker[TLocal](Int32 fromInclusive, Int32 toExclusive, ParallelOptions parallelOptions, Action`1 body, Action`2 bodyWithState, Func`4 bodyWithLocal, Func`1 localInit, Action`1 localFinally)
    at System.Threading.Tasks.Parallel.ForEachWorker[TSource,TLocal](TSource[] array, ParallelOptions parallelOptions, Action`1 body, Action`2 bodyWithState, Action`3 bodyWithStateAndIndex, Func`4 bodyWithStateAndLocal, Func`5 bodyWithEverything, Func`1 localInit, Action`1 localFinally)
    at System.Threading.Tasks.Parallel.ForEachWorker[TSource,TLocal](IEnumerable`1 source, ParallelOptions parallelOptions, Action`1 body, Action`2 bodyWithState, Action`3 bodyWithStateAndIndex, Func`4 bodyWithStateAndLocal, Func`5 bodyWithEverything, Func`1 localInit, Action`1 localFinally)
    at System.Threading.Tasks.Parallel.ForEach[TSource](IEnumerable`1 source, ParallelOptions parallelOptions, Action`1 body)
    at Xamarin.Android.Tasks.Aapt2Compile.DoExecute()
    at Xamarin.Android.Tasks.Aapt2Compile.<Execute>b__15_0()
    at System.Threading.Tasks.Task.InnerInvoke()
    at System.Threading.Tasks.Task.Execute() [E:\A\_work\1214\s\bin\TestRelease\temp\BuildAMassiveApp\App1\App1.csproj]

One or more errors occurred is not helpful, and it looks like this
could happen in several places throughout our MSBuild tasks. The only
way to get a better error is to add a try-catch and manually log the
exception.

A couple extension methods can simplify where we use the PCL, and fix
this problem at the same time:

• Parallel.ForEach -> this.ParallelForEach():
  • Can setup ParallelOptions & CancellationToken
  • Adds a try-catch block that reports XA0000 on unhandled
    exceptions
  • Calls AsyncTask.Cancel() if there is an exception
• this.ParallelForEachWithLock passes an object lockObject to the
  inner method for any needed locking.
• Task.Run -> this.RunTask()
  • Sets up the CancellationToken

With these extension methods we can also drop this using in several
files:

using ThreadingTasks = System.Threading.Tasks;

It is a bit annoying to have to disambiguate an MSBuild task from a
threading task.

[jonathanpeppers]

Aha! the error is:

Unhandled exception: System.IndexOutOfRangeException: Index was outside the bounds of the array.
   at System.Collections.Generic.List`1.Add(T item)
   at Xamarin.Android.Tasks.Aapt2Compile.ProcessDirectory(ITaskItem resourceDirectory)

I think this should also throw; to abort the Parallel.ForEach otherwise I got this many errors:

Or maybe there is some usage of CancellationToken to cleanly exit.

[jonathanpeppers]

With the latest change to call Cancel() the errors are more reasonable:

We still got 5 errors, but were getting like 20 before.

[jonpryor]

@monojenkins build

[jonathanpeppers]

I think all the build failures are OK:

• Windows one is the AOT failures we have on master--should be fixed by Bump to mono/mono/2018-10@2264e1b30cc #2897
• the macOS failures all look network-related?