Remember to cancel lingering when a network again satsifies a NetworkRequest.

When WiFi's score drops and then comes back up we would previously linger
WiFi but forget to cancel the linger timeout, so 30s later WiFi would
unexpectedly tear down.  This was not completely fixed in 0cc1732c.

bug:18826162
Change-Id: I7bb4b99ec969099e9815f46d4c09253be71a29be