[Xamarin.Android.Build.Tasks] Replace Newtonsoft.Json usage with System.Text.Json

[pjcollins]

Context: #9226 (review)

We should be able to stop repacking or redistributing Newtonsoft.Json if we update the tasks which use it to instead use System.Text.Json. At a glance it looks like only the following tasks would need updating:

https://github.com/dotnet/android/blob/main/src/Xamarin.Android.Build.Tasks/Tasks/BuildAppBundle.cs
https://github.com/dotnet/android/blob/main/src/Xamarin.Android.Build.Tasks/Tasks/JavaDependencyVerification.cs
https://github.com/dotnet/android/blob/main/src/Xamarin.Android.Build.Tasks/Utilities/MamJsonParser.cs