Mini cleanup to stream.ActorMaterializer (#25866)

Do it once, instead of in all 4 call sites.

akka-stream/src/main/scala/akka/stream/ActorMaterializer.scala

@@ -67,14 +67,13 @@ object ActorMaterializer {
       FlowNames(system).name.copy(namePrefix))
   }
 
-  private def actorOfStreamSupervisor(materializerSettings: ActorMaterializerSettings, context: ActorRefFactory, haveShutDown: AtomicBoolean) =
+  private def actorOfStreamSupervisor(materializerSettings: ActorMaterializerSettings, context: ActorRefFactory, haveShutDown: AtomicBoolean) = {
+    val props = StreamSupervisor.props(materializerSettings, haveShutDown)
     context match {
-      case s: ExtendedActorSystem ⇒
-        s.systemActorOf(StreamSupervisor.props(materializerSettings, haveShutDown).withDispatcher(materializerSettings.dispatcher), StreamSupervisor.nextName())
-
-      case a: ActorContext ⇒
-        a.actorOf(StreamSupervisor.props(materializerSettings, haveShutDown).withDispatcher(materializerSettings.dispatcher), StreamSupervisor.nextName())
+      case s: ExtendedActorSystem ⇒ s.systemActorOf(props, StreamSupervisor.nextName())
+      case a: ActorContext        ⇒ a.actorOf(props, StreamSupervisor.nextName())
     }
+  }
 
   /**
    * Scala API: Creates an ActorMaterializer that can materialize stream blueprints as running streams.
@@ -101,8 +100,9 @@ object ActorMaterializer {
       system,
       materializerSettings,
       system.dispatchers,
-      system.systemActorOf(StreamSupervisor.props(materializerSettings, haveShutDown)
-        .withDispatcher(materializerSettings.dispatcher), StreamSupervisor.nextName()),
+      system.systemActorOf(
+        StreamSupervisor.props(materializerSettings, haveShutDown),
+        StreamSupervisor.nextName()),
       haveShutDown,
       FlowNames(system).name.copy(namePrefix))
   }

akka-stream/src/main/scala/akka/stream/impl/ActorMaterializerImpl.scala

@@ -152,6 +152,7 @@ private[akka] class SubFusingActorMaterializerImpl(val delegate: ExtendedActorMa
 @InternalApi private[akka] object StreamSupervisor {
   def props(settings: ActorMaterializerSettings, haveShutDown: AtomicBoolean): Props =
     Props(new StreamSupervisor(settings, haveShutDown)).withDeploy(Deploy.local)
+      .withDispatcher(settings.dispatcher)
   private[stream] val baseName = "StreamSupervisor"
   private val actorName = SeqActorName(baseName)
   def nextName(): String = actorName.next()

akka-stream/src/main/scala/akka/stream/impl/PhasedFusingActorMaterializer.scala

@@ -71,8 +71,9 @@ import akka.util.OptionVal
     val system = actorSystemOf(context)
     val materializerSettings = ActorMaterializerSettings(system)
 
-    val streamSupervisor = context.actorOf(StreamSupervisor.props(materializerSettings, haveShutDown)
-      .withDispatcher(materializerSettings.dispatcher), StreamSupervisor.nextName())
+    val streamSupervisor = context.actorOf(
+      StreamSupervisor.props(materializerSettings, haveShutDown),
+      StreamSupervisor.nextName())
 
     PhasedFusingActorMaterializer(
       system,