WifiService: Wifi power management change

Put Wifi into an idle state immediately if the screen is turned off and the Wifi interface has no IP address.
We will continue to keep Wifi up for 15 minutes in the case where the screen is turned off when Wifi is fully connected.
This will allow us to go into a low power mode faster when Wifi is not actively being used.
It also avoids bringing up Wifi if the user just turns on the screen for a few seconds to check the clock, etc.

Fixes bug b/1736920

Signed-off-by: Mike Lockwood <lockwood@android.com>