Revert "Changed ConnectivityService#getServiceInstance so its a singleton."

This reverts commit 28961cc0 which causes this exception:

E/SystemServer( 2653): Failure starting Connectivity Service
E/SystemServer( 2653): java.lang.IllegalMonitorStateException: object not locked by thread before wait()
E/SystemServer( 2653):  at java.lang.Object.wait(Native Method)
E/SystemServer( 2653):  at java.lang.Object.wait(Object.java:358)
E/SystemServer( 2653):  at com.android.server.ConnectivityService$ConnectivityThread.getServiceInstance(ConnectivityService.java:186)
E/SystemServer( 2653):  at com.android.server.ConnectivityService.getInstance(ConnectivityService.java:200)
E/SystemServer( 2653):  at com.android.server.ServerThread.run(SystemServer.java:262)

Change-Id: Idd139e426e61a1913b0bf343e2f0a0b1990ba8bb