Move most of cvd/cli/commands/start.cpp into a private namespace

`NewCvdStartCommandHandler` is the only symbol exposed by the header, so
the others shouldn't be part of the `cuttlefish` namespace.

Author: Databean

base/cvd/cuttlefish/host/commands/cvd/cli/commands/start.cpp

@@ -246,8 +246,6 @@ Result<std::unique_ptr<OperatorControlConn>> PreregisterGroup(
   return operator_conn;
 }
 
-}  // namespace
-
 class CvdStartCommandHandler : public CvdCommandHandler {
  public:
   CvdStartCommandHandler(InstanceManager& instance_manager)
@@ -682,6 +680,8 @@ Result<std::string> CvdStartCommandHandler::DetailedHelp(
   return kDetailedHelpText;
 }
 
+}  // namespace
+
 std::unique_ptr<CvdCommandHandler> NewCvdStartCommandHandler(
     InstanceManager& instance_manager) {
   return std::unique_ptr<CvdCommandHandler>(