Move notif volume pref on top of increasing ring pref

Change Iafc79a57651db327a100e0f1723b62f442b638be added two
increasing ringtone-related preferences between the notification
volume preference and the ring volume preference.

However, the notification volume preference should be grouped
together with the media, alarm, and ring volume preferences.

Hence, move the notification volume preference back to where
it should be.

Change-Id: I138e6f78b6c231643d86746858e0cbdfcb0942eb