ModemConfig: Set notification channel blockable

android.app.NotificationChannel#mSystemBlockable is false by default.
If the app was built with system signature the notification channel cannot be blocked.

Change-Id: I24133042654bf36a18d0da8b273220e9d7e9d5d3

ModemConfig/src/com/sony/opentelephony/modemconfig/ModemConfigService.kt

@@ -228,6 +228,7 @@ class ModemConfigService : Service() {
                 NOTIFICATION_CHANNEL_ID,
                 resources.getString(R.string.notification_channel_configuration_info),
                 NotificationManager.IMPORTANCE_MIN)
+        channel.setBlockable(true)
         notificationManager.createNotificationChannel(channel)
 
         // Our callback is invoked once on .add too; no need to run the contents manually at startup