VolumeDialog: Display default row when active row is notification

Commit "frameworks: Add unlinked ringtone and notification volumes"
introduced STREAM_NOTIFICATION. However, this stream type, although
marked as important, was not added to the list in shouldBeVisibleH.
As a result, the volume panel behavior of STREAM_NOTIFICATION deviates
from other user-facing streams like media, call and ring.

This change adds STREAM_NOTIFICATION so the behavior becomes consistent.

Change-Id: I092c5bf0ae8cbee85af6adfa0da308dfdb60e66a
Signed-off-by: Jesse Chan <jc@lineageos.org>