Move the net transition wakelock to ConnService.

When the default network goes down we lose the wake-on-incoming-data capability
until the new net is brought up and apps rebuild their connections.  We fixed this
in Wifi, but it's a general connectivity issue, not a wifi issue so moving the
mechanism to connecitivty so other networks can use it.

bug:2734419
Change-Id: I39b5d825eb6b548bd9bb8f179b89254f4db53147