Fix extra data in cache

Reverting the revert.
The original CL(commit a666d74d) had a 
bug in it. It was calling accountExistsCacheLocked(), while holding 
accounts.cacheLock. The function accountExistsCacheLocked, was in turn
calling into getUserAccounts, which acquires mUsers. And this causes
trouble. mUsers is a a lock on all accounts, and hence calling it after
holding accounts.cachelock is calling for trouble. Since the locks are 
acquired in other order, it causes a potential deadlock issue, which we 
discovered later on.

That bug was fixed by commit 0e592733.
We already have useraccount object and should reuse it.

This reverts commit 27d0e1fd. This 
commit was unneeded as the bug had been fixed by that time.

Change-Id: I5328c31fd485fd2c1c652cd0e7c2c4bded38a5fd