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