Reverse engineering on Android is REALLY easy ! You can't prevent that. You should not store any sensitive informations in your APK because someone could find them easily.
You should use asymmetric encryption if you want to store something on the user device.
It's possible to hide some data in your code like a symmetric encryption key but it will be found in few minutes if someone want to find it. (and few seconds if you put it in assets folder...)
EDIT If you want to put a symmetric encryption key in your code, don't set it like :
String myKey = "myEncryptionKey";
byte[] key = myKey.getBytes();
because a reverse engineer is able to list all strings in your apk with a single command... So use something like :
StringBuilder sb = new StringBuilder();
sb.append(m);
sb.append(y);
...
byte[] key = sb.toString().getBytes();
or
byte[] key = Base64.decode("esfas09f8as90f8").getBytes();