OP_ACTIVATE_VPN should not map to DISALLOW_CONFIG_VPN

They are two different things.
OP_ACTIVATE_VPN means can we allow a vpn app to establish without user
interaction while DISALLOW_CONFIG_VPN means can the user modify vpn
config.

Testing:
1. Turn on DISALLOW_CONFIG_VPN, user cannot modify vpn config through
   setting
2. Turn on DISALLOW_CONFIG_VPN, Dpm.setAlwaysOnVpn works.
3. Reboot after 2, vpn established automatically.

Bug: 29086229
Change-Id: I24899d6c7f0dd62bf441a44c6ee66fee35973a11