!str #16993 Separate scaladsl/javadsl for FlattenStategy

`abstract class FlattenStrategy` was used in both javadsl and scaladsl, but the concrete concat
for the javadsl was in javadsl.FlattenStrategy and the concrete concat for the scaladsl is in
stream.FlattenStrategy.

Now there are separate FlattenStategy in scaladsl and javadsl packages and conversion
as we have in other places.

* replace JavaConverters with explicit methods

* remove asJava/asScala for FlattenStrategy

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -3,6 +3,7 @@
  */
 package akka.stream.scaladsl
 
+import akka.stream.javadsl
 import akka.stream.impl.Stages.{ MaterializingStageFactory, StageModule }
 import akka.stream.{ SourceShape, Inlet, Outlet }
 import akka.stream.impl.StreamLayout.{ EmptyModule, Module }
@@ -152,6 +153,9 @@ final class Source[+Out, +Mat](private[stream] override val module: Module)
   override def withAttributes(attr: OperationAttributes): Repr[Out, Mat] =
     new Source(module.withAttributes(attr).wrap())
 
+  /** Converts this Scala DSL element to it's Java DSL counterpart. */
+  def asJava: javadsl.Source[Out, Mat] = new javadsl.Source(this)
+
 }
 
 object Source extends SourceApply {