Attempt to fix concurrent modification issue in QuickstepWidgetHolder

Test: N/A
Bug: 275100497
Change-Id: I5db18ff38aa280c2a9957c457ac9565570933513
diff --git a/quickstep/src/com/android/launcher3/uioverrides/QuickstepWidgetHolder.java b/quickstep/src/com/android/launcher3/uioverrides/QuickstepWidgetHolder.java
index b318100..91cefb5 100644
--- a/quickstep/src/com/android/launcher3/uioverrides/QuickstepWidgetHolder.java
+++ b/quickstep/src/com/android/launcher3/uioverrides/QuickstepWidgetHolder.java
@@ -197,7 +197,7 @@
     @Override
     public void addProviderChangeListener(
             @NonNull LauncherWidgetHolder.ProviderChangedListener listener) {
-        mProviderChangedListeners.add(listener);
+        MAIN_EXECUTOR.execute(() -> mProviderChangedListeners.add(listener));
     }
 
     /**
@@ -207,7 +207,7 @@
     @Override
     public void removeProviderChangeListener(
             LauncherWidgetHolder.ProviderChangedListener listener) {
-        mProviderChangedListeners.remove(listener);
+        MAIN_EXECUTOR.execute(() -> mProviderChangedListeners.remove(listener));
     }
 
     /**