UPL (Unreal Plugin Language) is the new name now it has been generalized for iOS plist modification as well.
You can use it to make additions to GameActivity.java during packaging from a plugin. Adding a new class to GameActivity can be done with gameActivityClassAdditions section.
Or, you can have the UPL file copy your own java files to be compiled either in the src build path, or as an AAR to extract by adding it dynamically to the aar-imports.txt, or by directly copying the library into JavaLibs in the prebuildCopies section.
You can find documentation for UPL in the comment block at the top of the UnrealPluginLanguage.cs source file on GitHub.