Fix system notifications when visual signals flag is on

This mimics the previous notification behaviour, so adjust the
FileOperationService tests (that test these notifications) to work with
the flag on.

Bug: 411000762
Test: atest -c 'DocumentsUIGoogleTests:com.android.documentsui.services.FileOperationServiceTest'
Flag: com.android.documentsui.flags.visual_signals_ro
Change-Id: I75abe6c41c432027c10f28b8e28f204715c1f240