Android: Implement JNI_OnLoad()

Android: Implement JNI_OnLoad()

When building for Android, chances are that gstreamer is going to be
loaded from Java using System.loadLibrary(). In that case we can
initialize GStreamer (including static plugins), redirect log functions,
etc.

This code is copied from cerbero because it can be used with
gstreamer-full-1.0 too. Cerbero needs to be adapted to drop that code
and generate gst_init_static_plugins() function.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/617>