facebook/facebook-android-sdk ? reverse-engineered prompt

Build me an Android SDK that lets app developers add Facebook features to their apps, with separate modules for login, sharing, Messenger, App Links, analytics, Graph API, and marketing.

I want it organized so people can include only the pieces they need, or pull in everything at once. Make sure the library is usable from a normal Android app, follows the usual Android build setup, and includes a few sample apps or examples that show how to wire it up. Keep the code clean and modular, and make sure initialization only happens in the main app process.

If anything depends on current Facebook platform behavior or Android setup details, look up the latest docs online first so the implementation matches what developers actually need today.