Settings: Add switch for linked ring and media notification volumes

Author: Dan Pasanen <dan.pasanen@gmail.com>
Date:   Wed Oct 30 09:22:30 2019 -0500

    VolumePanel: fix showing notification volume slider when unlinked

    * volume_link_notification lives in Settings.Secure not LineageSettings.Secure
    * Set max panel slices to 6. We could have up to 5 volumes to control and
      MEDIA_OUTPUT_INDICATOR_SLICE_URI counts as one slice as well.

    Change-Id: I0a4c7db1c92e778fba669931259cd52164b1df65

Change-Id: I900af398f566a5415c44fe883cc214985f4e9cd4