ObservableTunnel: Don't recurse in getConfig

The correct way to retrieve the value inside a getter/setter is to use `field` to ensure
you don't invoke the getter while inside the getter and trigger a stack overflow

Signed-off-by: Harsh Shandilya <me@msfjarvis.dev>

ui/src/main/java/com/wireguard/android/model/ObservableTunnel.kt

@@ -65,9 +65,9 @@ class ObservableTunnel internal constructor(
     @get:Bindable
     var config = config
         get() {
-            if (config == null)
+            if (field == null)
                 manager.getTunnelConfig(this).whenComplete(ExceptionLoggers.E)
-            return config
+            return field
         }
         private set