Fix service name for VPN_MANAGEMENT_SERVICE.

aosp/1574082 added sepolicy for a "vpnmanager" system service
which is being introduced to move code from ConnectivityService
to a new VpnManagerService.

Unfortunately that CL missed the fact that "vpn_management" is
already a service name and present in the public API since R.
Instead of adding another service name, use the existing service
name to lessen confusion. It is difficult to avoid confusion
entirely because there was already a public class called
VpnService when the VpnManager class was added to the public API
surface.

Bug: 173331190
Test: builds, boots, "dumpsys vpn_management" throws no errors
Change-Id: I4ab188ef62592aac167ba1f7b586accc882815e8
3 files changed