Prevent app icons blinking after a user switch.

This CL moves OverviewProxyService from onUserChanged to
onUserChanging, to allow the jank to happen earlier while the user
switch is happening (i.e. while the screen is frozen),

Bug: 275349347
Test: Visual (screen recordings attached to the bug)
Change-Id: I8a5c2f56ff5d94f8c3754acac410f99675a3e1a2

packages/SystemUI/src/com/android/systemui/recents/OverviewProxyService.java

@@ -112,6 +112,7 @@ import java.io.PrintWriter;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
+import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.Executor;
 import java.util.function.Supplier;
 
@@ -445,6 +446,10 @@ public class OverviewProxyService implements CallbackController<OverviewProxyLis
             notifySystemUiStateFlags(mSysUiState.getFlags());
 
             notifyConnectionChanged();
+            if (mLatchForOnUserChanging != null) {
+                mLatchForOnUserChanging.countDown();
+                mLatchForOnUserChanging = null;
+            }
         }
 
         @Override
@@ -499,11 +504,14 @@ public class OverviewProxyService implements CallbackController<OverviewProxyLis
         }
     };
 
+    private CountDownLatch mLatchForOnUserChanging;
     private final UserTracker.Callback mUserChangedCallback =
             new UserTracker.Callback() {
                 @Override
-                public void onUserChanged(int newUser, @NonNull Context userContext) {
+                public void onUserChanging(int newUser, @NonNull Context userContext,
+                        CountDownLatch latch) {
                     mConnectionBackoffAttempts = 0;
+                    mLatchForOnUserChanging = latch;
                     internalConnectToCurrentUser("User changed");
                 }
             };