Channel settings updates

- Update master switch preference to store enabled state in case
it's set before the view holder is bound
- Update master switch preference to prevent disabled switches
from being toggled
- Show an importance summary for max importance
- properly handle null notification sounds

Change-Id: I395b95b76d2a3c1c94b41d1c2720bb0cf1cae917
Fixes: 36920159
Fixes: 37421928
Fixes: 36939825
Test: RunSettingsRoboTests & manual

src/com/android/settings/notification/AppNotificationSettings.java

@@ -161,7 +161,7 @@ public class AppNotificationSettings extends NotificationSettingsBase {
                     final NotificationChannel channel = channels.get(i);
                     MasterSwitchPreference channelPref = new MasterSwitchPreference(
                             getPrefContext());
-                    channelPref.setDisabledByAdmin(mSuspendedAppsAdmin);
+                    channelPref.setSwitchEnabled(mSuspendedAppsAdmin == null && !mAppRow.systemApp);
                     channelPref.setKey(channel.getId());
                     channelPref.setTitle(channel.getName());
                     channelPref.setChecked(channel.getImportance() != IMPORTANCE_NONE);