fix: Discard initial selection for single select

Workaround existing logic that dismisses the dialog if `allowMuliple` is
true and selection is not empty. The existing logic allows for quickly
selecting a single media item, but inhibits the ability to showcase a
pre-existing selection for single-select contexts. This diverges from
the web experience, but is the easiest path forward to avoid a
significant refactor of the logic that would impact all other existing
use cases--Gutenberg Mobile, Aztec, etc.

Author: dcalhoun

WordPress/Classes/ViewRelated/Media/SiteMedia/Controllers/SiteMediaCollectionViewController.swift

@@ -175,7 +175,7 @@ final class SiteMediaCollectionViewController: UIViewController, NSFetchedResult
         self.allowsMultipleSelection = allowsMultipleSelection
         self.isSelectionOrdered = isSelectionOrdered
 
-        if let selectedMedia = initialSelection {
+        if let selectedMedia = initialSelection, allowsMultipleSelection {
             setInitialSelection(selectedMedia)
         } else {
             deselectAll()