Split Android SDK2 into several sub-modules

The integration of the RustSDK is not easy because the arch of the crypto code is a bit messy.

We need to split the Matrix SDK into several sub modules:

- 1 module for client server API, including the model
- 1 module for authentication
- 1 module for crypto (to ease the future Rust binding)
- 1 module for the sync, to be plugged by other modules: crypto, db, etc.
- 1 top level module with main SDK objects: session, room, etc., as entry point of the SDK

Open for discussion...

Interesting feedbacks: https://github.com/MobileNativeFoundation/discussions/discussions/72