Fix NotificationManagerTest for HSUM.

In this case, the user-id should always be 0.

Bug: 417555342
Test: atest FrameworksUiServicesNotificationTests:com.android.server.notification.NotificationManagerServiceTest#testResolveNotificationUid_androidAllowed[EMPTY] -- --abi x86_64
Flag: EXEMPT bugfix
Change-Id: Ie1506fb2c52f2951e1402284340f08473c102e30