Provide access to Shell's apply token

This enables other processes (launcher) to share
the token so that the order of Transaction.apply()
matches the order that surfaceflinger actually
applies them.

Bug: 242193885
Test: quick-switch between apps repeatedly.
Change-Id: Id34e1ba8b59e0c82201d96ca65e430ce9696fd19

libs/WindowManager/Shell/src/com/android/wm/shell/transition/IShellTransitions.aidl

@@ -34,4 +34,9 @@ interface IShellTransitions {
      * Unregisters a remote transition handler.
      */
     oneway void unregisterRemote(in RemoteTransition remoteTransition) = 2;
+
+    /**
+     * Retrieves the apply-token used by transactions in Shell
+     */
+    IBinder getShellApplyToken() = 3;
 }

libs/WindowManager/Shell/src/com/android/wm/shell/transition/Transitions.java

@@ -952,6 +952,11 @@ public class Transitions implements RemoteCallable<Transitions> {
                         transitions.mRemoteTransitionHandler.removeFiltered(remoteTransition);
                     });
         }
+
+        @Override
+        public IBinder getShellApplyToken() {
+            return SurfaceControl.Transaction.getDefaultApplyToken();
+        }
     }
 
     private class SettingsObserver extends ContentObserver {