At the time Java supported applets and webstart, a special mechanism for launching various applications in one JVM was used to reduce memory usage and each application was isolated from each other.

This isolation was implemented via ThreadGroups where each application created its own thread group and all data for the application was stored in the context of its own group.

Some parts of the JDK used ThreadGroups directly, others used special classes like sun.awt.AppContext.

Such sandboxing became useless since the plugin and webstart are no longer supported and were removed in jdk11.

Note that this is just a first step for the overall cleanup, here is my plan:
1. Cleanup the usage of AppContext in the “java.util.logging.LogManager" class in the java.base module.
2. Cleanup the usage of TheadGroup in the JavaSound
3. Cleanup the usage of TheadGroup in the JavaBeans
4. Cleanup the usage of AppContext in the Swing
5. Cleanup the usage of AppContext in the AWT
6. Delete the AppContext class.

The current PR is for the first step. So this is the request to delete the usage of AppContext in the LogManager and related tests.