This is meant only to send download progress for any URL (e.g. index updates, APKs, etc). This also keeps this class pure Java so that classes that use ProgressListener can be tested on the JVM, without requiring an Android device or emulator.
Completes the hash computation by performing final operations such as padding and returns the resulting hash as a hex string. The digest is reset after this call is made, so call this only once and hang on to the result.