Draft: medialibrary: build the JNI part via gradle

So it can be edited and debugged within Android Studio.

Draft on top of !2359.

• By default the medialibrary is built locally, except in --release and --signrelease builds where it's using the AAR (-PaarMediaLib option in gradle).
• By default it's using the C++ shared STL (as before). It can be switched to static in the build script as before and with the gradle/Android Studio option: -PstaticSTL
• The gradle option vlcLibVariant that could set a single architecture to build
• A new gradle option to select multiple architectures to build is also possible: -PvlcLibABI=arm64-v8a,x86_64, in that case the vlcLibVariant option has no effect
• Only architectures where the libvlc.so and libmedialibrary.a are available will be built

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...