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