Switch to ~control for control channel

app/src/main/java/io/heckel/ntfy/msg/ApiService.kt

@@ -157,6 +157,9 @@ class ApiService {
 
     companion object {
         private const val TAG = "NtfyApiService"
+
+        // These constants have corresponding values in the server codebase!
+        const val CONTROL_TOPIC = "~control"
         const val EVENT_MESSAGE = "message"
         const val EVENT_KEEPALIVE = "keepalive"
     }

app/src/main/java/io/heckel/ntfy/ui/MainActivity.kt

@@ -117,7 +117,7 @@ class MainActivity : AppCompatActivity(), ActionMode.Callback, AddFragment.Subsc
         notifier!!.createNotificationChannels()
 
         // Subscribe to control Firebase channel (so we can re-start the foreground service if it dies)
-        messenger.subscribe("~keepalive")
+        messenger.subscribe(ApiService.CONTROL_TOPIC)
 
         // Background things
         startPeriodicPollWorker()