Support android versions 2.0+ (level 5 and up)

Small change to support older android versions. 
The problem is that the getbytes(Charset charset) method was only added on API level 9.

I've tested on a ECLAIR_MR1 phone and is working.
According to the android documentation the change is completely safe:

    public byte[] getBytes ()
    Returns a new byte array containing the characters of this string encoded using the system's default charset.

    The behavior when this string cannot be represented in the system's default charset is unspecified. 
    In practice, when the default charset is UTF-8 (as it is on Android), all strings can be encoded.

Android lint is also happy with the change.