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/ActivityTaskManagerService.java b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
index ac1dbc4..facfbed 100644
--- a/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
+++ b/services/core/java/com/android/server/wm/ActivityTaskManagerService.java
@@ -2042,8 +2042,8 @@
     public int getDisplayId(IBinder activityToken) throws RemoteException {
         synchronized (mGlobalLock) {
             final ActivityStack stack = ActivityRecord.getStackLocked(activityToken);
-            if (stack != null && stack.mDisplayId != INVALID_DISPLAY) {
-                return stack.mDisplayId;
+            if (stack != null && stack.getDisplayId() != INVALID_DISPLAY) {
+                return stack.getDisplayId();
             }
             return DEFAULT_DISPLAY;
         }
@@ -3201,7 +3201,7 @@
 
                 final ActivityStack stack = r.getActivityStack();
                 final Task task = stack.createTask(
-                        mStackSupervisor.getNextTaskIdForUserLocked(r.mUserId), ainfo, intent,
+                        mStackSupervisor.getNextTaskIdForUser(r.mUserId), ainfo, intent,
                         null /* voiceSession */, null /* voiceInteractor */, !ON_TOP);
                 if (!mRecentTasks.addToBottom(task)) {
                     // The app has too many tasks already and we can't add any more
@@ -4311,7 +4311,7 @@
 
         if (params.hasSetAspectRatio()
                 && !mWindowManager.isValidPictureInPictureAspectRatio(
-                        r.getActivityStack().mDisplayId, params.getAspectRatio())) {
+                        r.getDisplayId(), params.getAspectRatio())) {
             final float minAspectRatio = mContext.getResources().getFloat(
                     com.android.internal.R.dimen.config_pictureInPictureMinAspectRatio);
             final float maxAspectRatio = mContext.getResources().getFloat(