Following this, I tried to run the showcase app on my phone (the html and java versions both work fine). I get this error when I try to run mvn -Pandroid install though:
Failed to execute goal com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.6.0:generate-sources (default-generate-sources) on project playn-showcase-android: Execution default-generate-sources of goal com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.6.0:generate-sources failed: A required class was missing while executing com.jayway.maven.plugins.android.generation2:android-maven-plugin:3.6.0:generate-sources: Lorg/sonatype/aether/RepositorySystem;
Apparently this has something to do with some plugin being outdated/not working with maven 3.2 (which I'm running). I'm just not sure what I'm to do with that information. Maven is still relatively confusing for me (this doesn't seem to do anything too complicated, still it's a lot of boilerplate to digest), and I'm not sure what I can do to fix that problem there.
So, as it turns out hardcoding the version to 3.9.0-rc.1 kinda fixed it. I'd still like to know where the maven.android.version property comes from so I can do this properly.