Discard user data during sign out

Fixing issue #226 descibed here:
https://github.com/playgameservices/play-games-plugin-for-unity/issues/226

When signing out, mUserId and mUserDisplayName need to be reset so that they can be reinitialized in RetrieveUserInfo(). During sign in, RetrieveUserInfo() only sets mUserId and mUserDisplayName to current user values when they are null.