Add KEY_MIN_UDP_PORT_4500_NAT_TIMEOUT_SEC_INT in CarrierConfigManager

Add a new key to support keepalive customization for different
carriers to support dynamic keepalive. The default value will be
5 minutes when there is no customization.

Bug: 259000745
Test: m make update-api
Change-Id: I3e7950b925c9720afaed35dbfd09bb25e135ef44