Allow conversation channels to inherit canBypassDnd from their parent.

This change passes through a more correct value of fromTargetApp to PreferencesHelper.createNotificationChannel. The reason this fixes the inheritance of canBypassDnd for conversation channels is that that field cannot be modified by apps; but when the system creates a conversation channel, it should be able to inherit all its properties from its parent.

Bug: 253848209
Test: atest NotificationManagerServiceTest, atest NotificationManagerTest (CTS)
Change-Id: I407a239376afac18afd6c2cf2471a0e5399253a3