To anyone else who ended up here while searching for info on how to decode a string encoded with Base64.encodeBytes(), here was my solution:
// encode
String ps = "techPass";
String tmp = Base64.encodeBytes(ps.getBytes());
// decode
String ps2 = "dGVjaFBhC3M=";
byte[] tmp2 = Base64.decode(ps2);
String val2 = new String(tmp2, "UTF-8");
Also, I'm supporting older versions of Android so I'm using Robert Harder's Base64 library from http://iharder.net/base64