proguard: fix extends/implements warning

proguard.cfg

@@ -35,7 +35,7 @@
 -keep public class com.jazzido.PacketDroid.AudioBufferProcessor {
 	public void callback(byte[]);
 }
--keepclassmembers class * extends com.jazzido.PacketDroid.PacketCallback {
+-keepclassmembers class * implements com.jazzido.PacketDroid.PacketCallback {
 	public void received(...);
 }