Remove special-casing of rubicon-java Python code

[paulproteus]

This allows apps to bundle their desired version into app_packages.

Testing performed

I built an app using this template

$ cat pyproject.toml| grep -C 2 template

[tool.briefcase.app.helloworld.android]
template = "/Users/paulproteus/projects/beeware/briefcase-android-gradle-template"
requires = [
    '/Users/paulproteus/projects/beeware/rubicon-java',

and I configured a custom rubicon-java library in requires, and I added a print statement to the custom rubicon-java library and both the following things happened.

• The app launched (demonstrating that rubicon-java exists), and

• The print statement executed, which I know because I found it in the log.

Here's a partial log printout.

06-14 17:11:24.706  4929  4946 D stdio   : Import rubicon...
06-14 17:11:25.100  4929  4946 D stdio   : hi asheesh! love, rubucon.java.jni
06-14 17:11:26.121  4929  4946 D stdio   : Python app launched & stored in Android Activity class

(pardon the typo of rubucon vs rubicon)