Draft: medialibrary: build the JNI part via gradle

So it can be edited and debugged within Android Studio.

Now with parts of !2247 to allow the selection between AAR and local build (local by default)

Now on top of !2266 (merged) so we don't use the local folders if we are using the AAR (on demand or for lack of locally built .so)

Draft on top of !2253 (merged).

We'll also need to double check we can get the unstripped version of the .so in release builds. And that release builds do strip the .so...