Fix getItemIdForLink in SiteLinkLookup to return ItemId objects

PS1: This needs some adjustements in the code. Only the implementations
and tests are updated in the first patch set.

PS2+: In the following patch sets, other classes are updated, too.

Change-Id: I67824712f5a95aa045da4e08cc18a673abc21c35
Merge "Fix getItemIdForLink in SiteLinkLookup to return ItemId objects"