Factor out lookup code in DefaultVoicemailNotifier.

In preparation for sharing code with the MissedCallNotifier, factor out
the potential common code into CallLogNotificationsHelper.
+ Turn CallLogNotificationsHelper into a singleton.
+ Copy over NewCallsQuery and NameLookupQuery classes.
+ If it is a voicemail with unknown number, display the string
  "unknown".

Bug: 22857261
Bug: 25020236
Change-Id: Ie181472bd4588466ec5f37c8cbacf023ba1a21c4