sip: fix SipTransport class

- use a std::unique_ptr to handle the PJSIP transport (exception-proof).
  Deleter calls pjsip_transport_shutdown() then pjsip_transport_dec_ref().
- prevent a valid construction with invalid PJSIP transport.
- use insert than map [] operator to not cause extra memory allocations.
- minor: postfix private class member with an underscore.
- minor: using-aliases prefered than typedef.
- minor: code indentation fixes.

Refs #64903

Change-Id: I1e78d119ca7291928d5148344be5c0258ca2c02e