Add support for deleting default keystore entry

[xdgimf]

Right now you can only delete the secretKey but not the default keystore entry. This is useful for app that has optional fingerprint feature where user can turn it off anytime. When the user turn off the fingerprint feature we should delete the secretKey and keystore entry.

Deleting the keystore entry is easy to implement but we need something more convenient. I think we should include it in this library.

Its already implemented in CipherProvider.

private static void removeKey(String keyName) throws KeyStoreException, CertificateException, NoSuchAlgorithmException, IOException {
  if (keyExists(keyName)) {
    KeyStore keyStore = KeyStore.getInstance(ANDROID_KEY_STORE);
    keyStore.load(null);
    keyStore.deleteEntry(keyName);
  }
}

I propose something like:

RxFingerprint.removeKey() // Default KeyName
RxFingerprint.removeKey("CUSTOMKEY") // Custom KeyName

What do you guys think?