Add notification channel settings.

If a user has set a preference at an app wide level, it cannot be
undone at a channel level, but channels can be more restrictive.

Tests: changing settings for apps and channels.

Change-Id: I7907b8179ffc24d68197c917f182e63d8e1ff7a7

src/com/android/settings/DefaultRingtonePreference.java

@@ -31,7 +31,7 @@ public class DefaultRingtonePreference extends RingtonePreference {
     private static final String TAG = "DefaultRingtonePreference";
 
     private int mUserId;
-    private Context mUserContext;
+    protected Context mUserContext;
 
     public DefaultRingtonePreference(Context context, AttributeSet attrs) {
         super(context, attrs);