src: android: split jni_OnLoad/OnUnload from the core

Create a separate shared object (libvlc_android.so) to provide the
javaVM support in VLC core and libvlc. The application is responsible
from loading the shared object and it will provide the JavaVM for the
different clients needing it.

The core now optionnally (with runtime detection) depends on the JVM
availability instead of needing it unconditionnally, which also expand
the usage of the core on Android.

In a static build like currently for the libvlcjni releases, the library
will be built as a static archive too, and the two symbols will be
integrated into the final shared object, leading to no specific changes
for clients except the addition of the library in the final object being
linked.