To answer the above question, let me introduce few concepts related to Android
OpenMAX
Android uses OpenMAX for codec interface. Hence all native codecs (hardware accelerated or otherwise) provide OpenMAX interface. This interface is used by StageFright(Player framework) for decoding media using codec
NDK
Android allows Java Applications to interact with underlying C/C++ native libraries using NDK. This requires using JNI (Java Native Interface).
Now coming to your question How to tap native decoder to decode raw video bitstream?
In Android 4.0 version and below, Android did not provide access to underlying video decoders at Java layer. You would need to write native code to directly interact with OMX decoder. Though this is possible, it is not trivial as it would need knowledge of how OMX works and how to map this OMX to application using NDK.
In 4.1 (Jelly Bean version), Android seems to provide access to hardware accelerated decoders at application level through JAVA APIs. More details about new APIs at http://developer.android.com/about/versions/android-4.1.html#Multimedia