google: Only request the email field when getting the user's identity

Currently, a lot of personal information was being received, including gender and profile picture. The email is the only thing that's really used, for the account's identity and presentation identy, and the rest was being simply thrown away. Therefore, avoid receiving them in the first place.

Verified via the Google APIs Explorer and the Google OAuth 2.0 Playground:

• https://developers.google.com/apis-explorer/#p/oauth2/v2/oauth2.userinfo.get
• https://developers.google.com/oauthplayground/