Pre-cleanup before unifying Task and Stack (82/n)

- Removed Task#getTaskStack() and Task#mStack. Changed callers to use getStack().
- Made ActivityStack#mDisplayId private and changed callers to use getDisplayId.
- Switch stack id assignment to use getNextTaskIdForUser() so there isn't dups. with the ids.
- Made ActivityRecord#mTaskOverlay private.

Bug: 80414790
Test: Existing tests pass.
Change-Id: I47716f1fdac4cab21eed5a69f91e765f070e7550
diff --git a/services/core/java/com/android/server/wm/WindowContainer.java b/services/core/java/com/android/server/wm/WindowContainer.java
index 5c02f46..62630a6 100644
--- a/services/core/java/com/android/server/wm/WindowContainer.java
+++ b/services/core/java/com/android/server/wm/WindowContainer.java
@@ -1323,12 +1323,12 @@
             if (includeOverlays) {
                 return getActivity((r) -> true);
             }
-            return getActivity((r) -> !r.mTaskOverlay);
+            return getActivity((r) -> !r.isTaskOverlay());
         } else if (includeOverlays) {
             return getActivity((r) -> !r.finishing);
         }
 
-        return getActivity((r) -> !r.finishing && !r.mTaskOverlay);
+        return getActivity((r) -> !r.finishing && !r.isTaskOverlay());
     }
 
     void forAllWallpaperWindows(Consumer<WallpaperWindowToken> callback) {