Add hooks for WatchedItemQueryService / ApiQueryWatchlist

In order for an extension to add data to ApiQueryWatchlist, we need to
provide a way to allow it to manipulate the database query made by
WatchedItemQueryService. We also need some hooks in ApiQueryWatchlist to
handle the marshalling of data to and from WatchedItemQueryService.

To better handle hooking, this also moves some of the continuation logic
from ApiQueryWatchlist to WatchedItemQueryService.

Bug: T147939
Change-Id: Ie45376980f92da964a579887b28175c00fd8f57e