Ringer toggle always affects system sounds

Background:
- When DND is on, users can allow "touch sounds" (system sounds) to bypass DND
- The user-visible ringer stream and toggle controls notification,
ringer, and system sounds

Previously, allowing system sounds to bypass DND overrode the ringer
from affecting the system sound stream which made the volume
functionality of touch sounds different from in/out of DND.

This fix ensures that DND always respects the fact that the ringer toggle and stream
controls notifications, ringer, and system sounds at all times (whether in/out of DND).

Test: atest ZenModeHelperTest
Test: atest ZenModeConfigTest
Test: atest AudioManagerTest
Test: atest AudioManagerTest#testPriorityOnlySystemDisallowedWithRingerMuted
Test: atest AudioManagerTest#testPriorityOnlySystemAllowed
Fixes: 141292611
Change-Id: I6e36097f18a93ce554b3fad7e1a2bd1057ac9e1d