Cache reference to stack objects we use a lot in WM.

This helps with fast access since we are no longer using stack ids
to look-up stack objects.

Test: Existing tests pass.
Test: go/wm-smoke
Change-Id: I98cb6f4eb1e6d6acb81b2077aee03265a075a227
diff --git a/services/core/java/com/android/server/wm/AppWindowToken.java b/services/core/java/com/android/server/wm/AppWindowToken.java
index 1b463c7..5d03493 100644
--- a/services/core/java/com/android/server/wm/AppWindowToken.java
+++ b/services/core/java/com/android/server/wm/AppWindowToken.java
@@ -1310,8 +1310,7 @@
 
                 // Notify the pinned stack upon all windows drawn. If there was an animation in
                 // progress then this signal will resume that animation.
-                final TaskStack pinnedStack =
-                        mDisplayContent.getStack(WINDOWING_MODE_PINNED);
+                final TaskStack pinnedStack = mDisplayContent.getPinnedStack();
                 if (pinnedStack != null) {
                     pinnedStack.onAllWindowsDrawn();
                 }