Fix TileStore's ability to swap spotlight speakers without a layout shift

This was regressed in 79c40f198c because of the overlooked renaming of the 'speaking' field to 'speaking$'.

src/state/CallViewModel.test.ts

@@ -675,6 +675,21 @@ test("spotlight speakers swap places", () => {
             },
           },
         );
+
+        // While we expect the media on tiles to change, layout$ itself should
+        // *never* meaningfully change. That is, we expect there to be no layout
+        // shifts as the spotlight speaker changes; instead, the same tiles
+        // should be reused for the whole duration of the test and simply have
+        // their media swapped out. This is meaningful for keeping the interface
+        // not too visually distracting during back-and-forth conversations,
+        // while still animating tiles to express people joining, leaving, etc.
+        expectObservable(
+          vm.layout$.pipe(
+            distinctUntilChanged(deepCompare),
+            debounceTime(0),
+            map(() => "x"),
+          ),
+        ).toBe("x"); // Expect just one emission
       },
     );
   });

src/state/TileStore.ts

@@ -118,10 +118,11 @@ export class TileStore {
  */
 export class TileStoreBuilder {
   private spotlight: SpotlightTileData | null = null;
-  private readonly prevSpotlightSpeaker =
+  private readonly prevSpotlightSpeaker: UserMediaViewModel | null =
     this.prevSpotlight?.media.length === 1 &&
-    "speaking" in this.prevSpotlight.media[0] &&
+    "speaking$" in this.prevSpotlight.media[0]
-    this.prevSpotlight.media[0];
+      ? this.prevSpotlight.media[0]
+      : null;
 
   private readonly prevGridByMedia: Map<
     MediaViewModel,
@@ -201,7 +202,7 @@ export class TileStoreBuilder {
       if (
         media === this.prevSpotlightSpeaker &&
         this.spotlight.media.length === 1 &&
-        "speaking" in this.spotlight.media[0] &&
+        "speaking$" in this.spotlight.media[0] &&
         this.prevSpotlightSpeaker !== this.spotlight.media[0]
       ) {
         const prev = this.prevGridByMedia.get(this.spotlight.media[0]);