Allow companion device mgrs channel access

- Callbacks when channels and groups are modified
- Allow them to read and update channels and groups

Test: runtest systemui-notification
Change-Id: Ie4d02bd4583f71f9faf27603bcc59a1ec0eeaf46
diff --git a/services/core/java/com/android/server/notification/ManagedServices.java b/services/core/java/com/android/server/notification/ManagedServices.java
index 0774779..000cf2e 100644
--- a/services/core/java/com/android/server/notification/ManagedServices.java
+++ b/services/core/java/com/android/server/notification/ManagedServices.java
@@ -154,6 +154,13 @@
 
     abstract protected void onServiceAdded(ManagedServiceInfo info);
 
+    protected List<ManagedServiceInfo> getServices() {
+        synchronized (mMutex) {
+            List<ManagedServiceInfo> services = new ArrayList<>(mServices);
+            return services;
+        }
+    }
+
     protected void onServiceRemovedLocked(ManagedServiceInfo removed) { }
 
     private ManagedServiceInfo newServiceInfo(IInterface service,