samples: Fix keep_alive_interval default in pjsip.conf.

Since ASTERISK_27978 the default is not off but 90 seconds. That change
happened because ASTERISK_27347 disabled the keep-alives in the bundled
PJProject and Asterisk should behave the same as before.

Change-Id: Ie63dc558ade6a5a2b969c30a4bd492d63730dc46