Remove volume_separate_notification flag

Fully enable the feature

Bug: b/255363741, b/38477228

Test: make DEBUG_ROBOLECTRIC=1 ROBOTEST_FILTER="NotificationVolumePreferenceControllerTest|SeparateRingVolumePreferenceControllerTest|RingerModeAffectedPreferenceControllerTest|SoundSettingsTest" RunSettingsRoboTests -j40
Change-Id: Iec490fc254f71b6461dbf9f9beb4a11645a4a497
10 files changed