Improve SeekBarVolumizer behavior with unlinked streams

- Fix notification stream enabling at 0 volume
- Fix the notification slider being set when moving ringer stream

Change-Id: If3705cc833c07f726f8ad7b1ec79a42e86e8ff22
Signed-off-by: Roman Birg <roman@cyngn.com>
(cherry picked from commit cdead087)