Adjust polling interval for auto on/off keepalives

The system polls TCP connections every 2 minutes for the auto
on/off keepalives. For some carriers, the NAT-T keepalive delay
is lower than this, which means it's possible a TCP connection
is opened seconds after the poll. This may cause connections
being disconnected because keepalive timed-out.

The minimal acceptable value should be the same as the keepalive
delay. But the system may take time to restart keepalive. Adjust
the delay of the polling alarm to be lower than the keepalive
delay to preserve some time for the system to restart the
keepalive.

Bug: 273411114
Test: atest FrameworksNetTests
Change-Id: I28f71ceb7b72d83e6e0f8f7d22d3d1e9dbdaa467