In projLib‘s build.gradle file, you’ll see a statement like this:
apply plugin: 'com.android.application'
which tells Gradle to build it as an application, generating an APK. If you change it to this:
apply plugin: 'com.android.library'
it will build as a library, generating an AAR, and it should work.
If you also need projLib to generate a separate APK, then you’ll have to do some refactoring to pull the common code that you need out into a third library module, and have both APKs depend on it.
Libraries aren’t allowed to set an applicationId, so if you see an error message to that effect, remove it from the library’s build script.