Implement NFC token handling

- Add NfcHelpers to remove boilerplate NFC handling
- Include otp-client library for communicating with otp tokens
- Add InternalMainActivity to show tokens stored in phone
- On tap show ExternalMainActivity that lists tokens store in the
  external hardware token
- Change action bar color and title to show that an external token is in
  use
- Extend AddActivity to add to external token when it is presented
- After scanning in NFCmode send user to AddActivity
- Extend DeleteActivity to remove from external token when it is presented
- Force refresh after leaving the ExternalMainActivity