Implement UserHistoryDictionary for each user account.

UserHistoryDictionary currently uses locale to determine the
UserHistoryDictionary on the filesystem. With this change we
use the account name as well. Thus each UserHistoryDictionary
would following the following spec:

UserHistoryDictionary.<locale>.<account>.dict.
In case no account is selected, we default to the existing
spec:
UserHistoryDictionary.<locale>.dict

Example
UserHistoryDictionary.en_US.testaccount@example.com.dict

Bug: 18104749
Change-Id: Iab031e166b55cf2ded68275a7e9be22475737b37