SIP: add SERVER_UNREACHABLE error code.

Let SipSession return it when UnknownHostException is caught.
Add DisconnectCause.SERVER_UNREACHABLE in Connection and have SipPhone report
it when receiving SERVER_UNREACHABLE from SipSession.

http://b/issue?id=3061691

Change-Id: I944328ba3ee30c0a9386e89b5c4696d4d9bde000
4 files changed