Discarding user ID and name during sign out

Fixing issue #226 described here:
https://github.com/playgameservices/play-games-plugin-for-unity/issues/226

1. Sign in with account_1
2. Sign out
3. Sign in with account_2

Methods GetUserId() and GetUserDisplayName() will still return ID and name for account_1 because mUserId and mUserDisplayName weren't reset to null during sign out. Notice that RetrieveUserInfo() will only set mUserId and mUserDisplayName to their corresponding values when mUserId == null and mUserDisplayName == null.