Make GDNative work on Android

The changes include work done to ensure that GDNative apps and Nim
integration specifically can run on Android. The changes have been
tested on our WIP game, which uses godot-nim and depends on several
third-party .so libs, and Platformer demo to ensure nothing got broken.

 - .so libraries are exported to lib/ folder in .apk, instead of assets/,
   because that's where Android expects them to be and it resolves the
   library name into "lib/<ABI>/<name>", where <ABI> is the ABI matching
   the current device. So we establish the convention that Android .so
   files in the project must be located in the folder corresponding to
   the ABI they were compiled for.

 - Godot callbacks (event handlers) are now called from the same thread
   from which Main::iteration is called. It is also what Godot now
   considers to be the main thread, because Main::setup is also called
   from there. This makes threading on Android more consistent with
   other platforms, making the code that depends on Thread::get_main_id
   more portable (GDNative has such code).

 - Sizes of GDNative API types have been fixed to work on 32-bit
   platforms.

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