res_pjsip: Update default keepalive interval to 90 seconds.

A change recently went in which disabled the built-in PJSIP
keepalive. This defaulted to 90 seconds and kept TCP/TLS
connections alive. Disabling this functionality has resulted
in a behavior change of not doing keepalives by default resulting
in TCP/TLS connections dropping for some people.

This change makes our default keepalive interval 90 seconds
to match the previous behavior and preserve it.

ASTERISK-27978

Change-Id: Ibd9a45f3cbe5d9bb6d2161268696645ff781b1d6