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/tests/wmtests/src/com/android/server/wm/DisplayContentTests.java b/services/tests/wmtests/src/com/android/server/wm/DisplayContentTests.java
index 0758eeb..73dd2df 100644
--- a/services/tests/wmtests/src/com/android/server/wm/DisplayContentTests.java
+++ b/services/tests/wmtests/src/com/android/server/wm/DisplayContentTests.java
@@ -126,7 +126,7 @@
         waitUntilHandlersIdle();
 
         exitingApp.mIsExiting = true;
-        exitingApp.getTask().getTaskStack().mExitingActivities.add(exitingApp);
+        exitingApp.getTask().getStack().mExitingActivities.add(exitingApp);
 
         assertForAllWindowsOrder(Arrays.asList(
                 mWallpaperWindow,