audio service: do not persist system enforced stream volume

Persisting STREAM_SYSTEM_ENFORCED index is not needed as its alias (STREAM_RING)
is persisted. This can also be problematic when the enforcement is active as it will
override current SYSTEM_RING persisted value given they share the same settings name
(due to aliasing).

Bug: 317938475
Test: repro steps in the bug
Change-Id: Id6cf47407bd81da45d39762cceeb4f10cf76476b