Add a Dictionary.isUserSpecific method.

As the name suggests, returns true if the Dictionary is of a type
that may use user-specific data, e.g., contacts.

Cherry-pick of I11e71286c7635f50ace156dcc5308dbe51268d4e

Change-Id: I08c3f22f0c34eaa71bbd06594314f3b411658c5c