Add system stream to isZenMuted

This CL addresses this
https://googleplex-android-review.git.corp.google.com/c/platform/frameworks/base/+/26256302/comment/c0b411c7_d034125a/
comment.

Flag: aconfig new_volume_panel TEAMFOOD
Test: atest NotificationsSoundPolicyInteractorTest
Fixes: 326440214
Change-Id: Ia64bc543601175675ded9b316d51ab50258b4296