Turn Flow into an interface and inject all its fields

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=137903095

java/google/registry/flows/picker/FlowPicker.java

@@ -91,7 +91,7 @@ import java.util.Set;
 public class FlowPicker {
 
   /** Marker class for unimplemented flows. */
-  private abstract static class UnimplementedFlow extends Flow {}
+  private abstract static class UnimplementedFlow implements Flow {}
 
   /** A function type that takes an {@link EppInput} and returns a {@link Flow} class. */
   private abstract static class FlowProvider {