Prompt to Google Authenticator app when setup QR code is scanned, instead of deferring the decision to user's phone

[polymorpher]

One user reported when she scanned the QR code using the camera app on her phone, she got redirected to Duo Mobile app instead of Google Authenticator. This is problematic for three reasons:

1. Duo Mobile app's clock is ~10 seconds off, often gives the user incorrect OTP code. This particular user reported that she "tried at least 5 times but the wallet also said the code is incorrect", so she gave up.
2. Duo Mobile doesn't display issuer correctly ("Third Party" instead of "Harmony").
3. Duo Mobile cannot export the entry as an QR code. The user cannot Restore the wallet from the authenticator.

[polymorpher]

Related: #55