Simplify the network transition wakelock.

Make it all internal to ConnectivityService - we know when a network
is lost, so grab a wakelock then.
Moves the call out of WifiStateMachine which was grabbing at bad times
like every dhcp renewal or corp-network roam.  These would always
go the full 1 minute and chew up battery.

bug:15595155
Change-Id: I80157a818cc149072cc7706d78c1e79c6e679ab3