Have paused keepalives keep their hardware slot

When a keepalive is paused, it should sit on its hardware
slot for this network to make sure that it is possible to
resume later, even if it means other keepalives can't be
started at the same time.

Test: update AutomaticOnOffKeepaliveTrackerTest for this
Fixes: 268149573
Fixes: 283886067
Change-Id: Ida325bdea198d751483a83ee5d9ec26e39812137