Fix some callwaiting/unhold issues

Fix the following issue:

• With Flash spec uk, the call waiting indication tone is not stopped after acceptance.
• In some un-hold scenarios, pjsip was sent indicate as un-hold-transfer only and did nothing if it is not a transfer, which makes the local chan unable to un-hold.

  A <-> B  
  A flash(to accept the call waiting, or just flash for a new dialing); B on hold  
  A on-hook  
  A ringing; B ringback tone  
  A off-hook; B got indicate as unhold-transfer and did nothing, ringback tone keeps going