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
diff --git a/core/java/android/provider/Settings.java b/core/java/android/provider/Settings.java
index dddbe78..5380cd6 100644
--- a/core/java/android/provider/Settings.java
+++ b/core/java/android/provider/Settings.java
@@ -4762,6 +4762,10 @@
         public static final String BAR_SERVICE_COMPONENT = "bar_service_component";
 
         /** @hide */
+        public static final String VOLUME_CONTROLLER_SERVICE_COMPONENT
+                = "volume_controller_service_component";
+
+        /** @hide */
         public static final String IMMERSIVE_MODE_CONFIRMATIONS = "immersive_mode_confirmations";
 
         /**