Allow sysui-managed remote volume controllers.

- Relax restriction on audio service calls that assume the volume
  ui is systemui, allow calls from a blessed component app.
- Blessed component app service saved in secure settings.
- SystemUI mediates requests to replace the volume dialog, prompts
  the user on activation.
- Show a low pri ongoing notification when the volume dialog is
  being replaced, to allow user restoration at any time.
- Replace the controller management code in VolumeUI to use a
  ServiceMonitor, backed by the new blessed app component setting.
- Add proper zen-related noman client wrappers, make avail to the
  registered volume controller.
- Everything is still @hidden, no api impact.

Bug: 19260237
Change-Id: Ie1383f57659090318a7eda737fdad5b8f88737d4