gen_jni: Overhaul build system

* Instead of building the JNI bindings inside libopenwebrtc{,_bridge}.so, build
  separate _jni.so libraries for those.
* Build the openwebrtc.jar and openwebrtc_bridge.jar files using autotools
owr/Makefile.am: Add Owr-0.1.gir.h to CLEANFILES