SIP: add the check for expiry time in Contact header.

There can be three expiry times in the same message header. We
choose the smaller value in Expires header and Contact header,
and then we obey the value defined in Min-Expires header. If
none of them is set, the default value is used.

Bug: 5178284

Change-Id: Ie9d4a48c93863e82e5197bb4a0db3f4fec56857c