The only exception is TRYING.
Also remove an unused import in SipSessionGroup.
http://b/issue?id=3021865
Change-Id: I160982b0c4b417362f1fb961217db90c3a585ce5
and fix how SipErrorCode.SERVER_ERROR is determinted from server response, not
from local exceptions.
http://b/issue?id=3041332
Change-Id: Idce67e29858d5c7573b98b7fa1fac074913d71d6