added OwrWindowHandle gir type + annotations to owr_window_registry
added gen_jni.py, which generates Java bindings for Android using GIR and JNI
added OwrBridge JNI implementation
modified build to generate, compile, and link JNI files when building for Android

Change-Id: I322f180f817db4ba92dbcd91446b876737a5c051
compile generated Java classes and create openwebrtc.jar and openwebrtc_bridge.jar when building for Android
owr/Makefile.am: Add various files to CLEANFILES, and do cleanup
configure: Detect Android-specific things using a configure option

SDK, NDK, JAVAC, JAR, etc.
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