PDL Pybind: Allow packets to contain packets

Bug: 143568835
Test: import bluetooth_packets_python3.hci_packets as hci_packets
      conn_handle = 23
      acl_packet = hci_packets.AclPacketBuilder(conn_handle,
          hci_packets.PacketBoundaryFlag.FIRST_NON_AUTOMATICALLY_FLUSHABLE,
          hci_packets.BroadcastFlag.POINT_TO_POINT, packet_to_send).Serialize()
      acl_packet
      # [23, 0, 3, 0, 20, 12, 0]
      payload = hci_packets.ReadLocalNameBuilder()
      payload.Serialize()
      # [20, 12, 0]
      conn_handle = 23
      acl_packet = hci_packets.AclPacketBuilder(conn_handle,
          hci_packets.PacketBoundaryFlag.FIRST_NON_AUTOMATICALLY_FLUSHABLE,
          hci_packets.BroadcastFlag.POINT_TO_POINT, payload)
      acl_packet.Serialize()
      # [23, 0, 3, 0, 20, 12, 0]
Change-Id: I4d5109d0f8d5ea4af4e935db4cdaf558a93ea74b