Current implementation assumes that OAuth provider always exposes
user name by using it for key in OAuth token cache. For some OAuth
providers (most notably Google OAuth provider) this is not the case.
Move from using user name to account id for token cache key. Postpone
populating the cache to the point when the authentication already took
place and use returned account id as cache key.
Bug: Issue 4627
Change-Id: I59f15b7c5ca8be6d52b59d21fac58cba88ba7fe3
d5b21545d7