Type checking and other features not working with Gradle project

I used this extension to work on a Maven project originally, and it worked very well. The main features I'm interested in are type-checking (getting a warning if I call a method with bad arguments, or if I call a method that doesn't exist, say) and import warnings (being told I need to import something before using it, and being able to add the import with a single click).

Now I'm working on a Gradle project (an Android project), and these features no longer work. When I open the developer console, I see this type of error every time I open a .java file:

Error: src/main/java/com/google/android/exoplayer/demo [in demo] is not on its project's build path

Environment

• Operating System: Ubuntu 18.10
• JDK version: 1.8
• Visual Studio Code version: 1.31.1
• Java extension version: 0.39.0

Start by reproducing the reported error when opening a Java file in the Gradle-based Android project, using the environment details in the issue. Trace how the project build path is detected and compare it with the working Maven case; done means type-checking and import warnings work without the build-path error.