Extend the time between WiFi energy samples

With bursty WiFi traffic, we end up sampling the WiFi controller's
energy data quite a lot. Extend the timeout so that we sample
once there has been no activity for 15 seconds.

Note: Once the WiFi radio goes down after being active, it can come back and be
active in less than 15 seconds, which means we may sample twice quickly.

Bug:21478443
Change-Id: I99081b664f8a33fef734bc55eef4d33ac297e83a