Store app & app_packages in separate directories

[paulproteus]

Close #2

Quality of this PR

This PR modifies a bunch of semi-unrelated Java code, and the Java code is somewhat messy. I'd prefer to leave this somewhat messy rather than block this change on Java cleanup.

Testing

First of all, I can verify this moves the user code into app:

$ unzip -l 'android/Hello World/app/build/outputs/apk/debug/app-debug.apk' | grep app         
Archive:  android/Hello World/app/build/outputs/apk/debug/app-debug.apk
        6  1980-00-00 00:00   META-INF/androidx.appcompat_appcompat.version
       50  1980-00-00 00:00   META-INF/app_debug.kotlin_module
      159  1980-00-00 00:00   assets/python/app/README
       10  1980-00-00 00:00   assets/python/app/click_button_receive_log-0.0.1.dist-info/INSTALLER
      225  1980-00-00 00:00   assets/python/app/click_button_receive_log-0.0.1.dist-info/METADATA
        0  1980-00-00 00:00   assets/python/app/click_button_receive_log/__init__.py
      166  1980-00-00 00:00   assets/python/app/click_button_receive_log/__main__.py
     2612  1980-00-00 00:00   assets/python/app/click_button_receive_log/app.py
   445986  1980-00-00 00:00   assets/python/app/click_button_receive_log/resources/click_button_receive_log.icns
    13871  1980-00-00 00:00   assets/python/app/click_button_receive_log/resources/click_button_receive_log.ico
    14040  1980-00-00 00:00   assets/python/app/click_button_receive_log/resources/click_button_receive_log.png
       73  1980-00-00 00:00   assets/python/app_packages/README

In addition, the app still launches properly. See screenshots.