b/72533250: Fix issue with limbo resolutions triggering incorrect manufactured deletes.

[Port of firebase/firebase-js-sdk#1014]

This fixes an issue occurring when a limbo target receives a documentUpdate,
then a global snapshot, and then a CURRENT. Because there was a global
snapshot before the CURRENT, WatchChangeAggregator has no pending document
updates and calls SyncEngine.remoteKeysForTarget to see if we previously got any
document from the backend for the target. See:
https://github.com/firebase/firebase-js-sdk/blob/6905339235ad801291edc696dd75a08e80647f5b/packages/firestore/src/remote/watch_change.ts#L422

Prior to this change, remoteKeysForTarget returned empty because
it relies on our Views to track the contents of the target, and we don't
have Views for limbo targets. Thus WatchChangeAggregator incorrectly
manufactures a NoDocument document update which deletes data from our
cache.

The fix is to have SyncEngine track the fact that we did indeed get
a document for the limbo resolution and return it from
remoteKeysForTarget.

Author: Michael Lehenbauer