System sounds get its own dnd toggle

- Added logic for streams to be muted by dnd (unassociated with ringer)

Test: runtest -x frameworks/base/services/tests/uiservicestests/src/com/android/server/notification/ZenModeHelperTest.java
Bug: 73362490
Change-Id: I1101e67282f7331497e91eb510d2bf4f1ea6b4b1