Centralize dependencies and migrate to submodules
Move `apache` and `axml` modules to a unified `external` directory and migrate libxposed API and service interfaces from Maven dependencies to Git submodules.
Included changes:
- Updated .gitmodules with new paths.
- Simplified core.yml CI workflow.
- Updated Gradle project references to use the new directory structure.
- Added documentation for external components.
To make `clangd` work properly, one may need to make symbolic links
```
cd daemon
ln -sf $PWD/build/tools/debug/arm64-v8a/compile_commands.json build
```
1. Use JDK 21
2. Update android plugins for JDK 21
3. Update gradle wrapper
The new R8 engine will change more class names than before, we thus
need to save those needed ones.
