[Feature]: auto-discover Java sources bundled in pip installed packages

If a pip-installable package ships Java source files alongside its Python code, p4a has no way to pick those up automatically. The only supported path is Buildozer's `android.add_src`, which requires the app developer to add files manually.

### Proposed solution

Right after the pip install stage in `build.py` scan the newly installed top level package directories for a fixed marker folder: `android_java/` (for example). If a package contains one, register its path and send to gradle.

Optionally also delete that folder as it's not required/used.

Original idea by @kuzeyron.

CC: @AndreMiras

Start in build.py immediately after the pip install stage and trace how source paths are registered and passed to Gradle. Implement discovery of top-level package android_java/ marker folders, then verify that discovered paths are registered for the Gradle build and consider the proposed folder cleanup.

Written by the indexing model from the issue text.