Don't renew the leases if mDhcpInfo.leaseDuration is not positive

Some routers might give out really long or static leases, which we
shouldn't renew. mDhcpInfo.leaseDuration is -1 when this happens,
so don't try to renew the lease if it's value is not positive.

Change-Id: I8cf404347e1122489886662e2f42b10f98917d39