Consider tasks with INVALID_DISPLAY as it's on DEFAULT_DISPLAY

- In the test environment, some main display tasks can have INVALID_DISPLAY (-1) as their displayId, we should consider them as main display tasks or else they'll be sorted to an unexpected section in RecentsView

Bug: 391311008
Test: TaplTestsQuickstep#testQuickSwitchFromHome
Flag: com.android.launcher3.enable_separate_external_display_tasks
Change-Id: I5a89d91e342557e114eae6796e153a160d729301
diff --git a/quickstep/src/com/android/quickstep/util/ExternalDisplays.kt b/quickstep/src/com/android/quickstep/util/ExternalDisplays.kt
index e1a8578..455b312 100644
--- a/quickstep/src/com/android/quickstep/util/ExternalDisplays.kt
+++ b/quickstep/src/com/android/quickstep/util/ExternalDisplays.kt
@@ -17,6 +17,7 @@
 package com.android.quickstep.util
 
 import android.view.Display.DEFAULT_DISPLAY
+import android.view.Display.INVALID_DISPLAY
 import com.android.systemui.shared.recents.model.Task
 
 /** Whether this displayId belongs to an external display */
@@ -25,7 +26,14 @@
 
 /** Returns displayId of this [Task], default to [DEFAULT_DISPLAY] */
 val Task?.displayId
-    get() = this?.key?.displayId ?: DEFAULT_DISPLAY
+    get() =
+        this?.key?.displayId.let { displayId ->
+            when (displayId) {
+                null -> DEFAULT_DISPLAY
+                INVALID_DISPLAY -> DEFAULT_DISPLAY
+                else -> displayId
+            }
+        }
 
 /** Returns if this task belongs tto [DEFAULT_DISPLAY] */
 val Task?.isExternalDisplay