Tethering: Use UsbManager to enable/disable RNDIS

Remove obsolete RNDIS support from NetworkManagementService

This change fixes USB tethering, which broke after the new USB gadget driver
changes went in.

Change-Id: Idd987fb2c39c40e81ebc92e6d8dae9fe77ed4ddc
Signed-off-by: Mike Lockwood <lockwood@android.com>