Switch OnSubscriptionsChangedListener to use existing Handler

Use the main Handler for DevicePolicyManagerService to post
the callback for OnSubscriptionsChanged to move away from
using the hidden (internal) handler.

Bug: 278814050
Test: atest FrameworksTelephonyTests
Test: atest SubscriptionManagerTest#testCreateSubscriptionChangedListenerWithoutLooper
Change-Id: I347408af8cd6cbcd3a25facc056c7c2541d5b428

services/devicepolicy/java/com/android/server/devicepolicy/DevicePolicyManagerService.java

@@ -24132,7 +24132,7 @@ public class DevicePolicyManagerService extends IDevicePolicyManager.Stub {
                 // When listener is added onSubscriptionsChanged gets called immediately for once
                 // (even if subscriptions are not changed) and later on when subscriptions changes.
                 subscriptionManager.addOnSubscriptionsChangedListener(
-                        mSubscriptionsChangedListener.getHandlerExecutor(),
+                        mHandler::post,
                         mSubscriptionsChangedListener);
             } finally {
                 mInjector.binderRestoreCallingIdentity(id);