Populate the interval millis for NattKeepalive

Currently the user-requested interval for NATT Keepalive
is not populated. The Radio is using a reasonable
default of 20s (which is why nobody noticed this), but
we need to populate this.

Bug: 110887063
Test: manually verified using test equipment
Change-Id: I57ae8086e5a3c797f6089d6a14f40007792036ed