Separate code in call state change callback
Separate code in call state change callback

PJSIP-related code is separated from AKSIPUserAgent-related
code. Because the callback can be called on any thread,
AKSIPUserAgent-related code is serialized on the main queue. This is
now done in one big block to be easier to understand.

mralexgray/SwitchBoard,inz/Telephone,thierryc/Telephone,nriley/Telephone,einfallstoll/Telephone,mobdim/Telephone