Allow any caller with CONTROL_VPN to disconnect existing VPN

When calling prepare(null, LEGACY_VPN, TYPE_VPN_SERVICE), the caller
wants to disconnect the current VPN. The current code checks to make
sure an IPC caller, and only an IPC caller, with the CONTROL_VPN
permission can do so.

But this doesn't allow for other processes in the system server (which
also have CONTROL_VPN permission) to do so. Expand the check to allow
those callers.

Bug: 284803285
Test: VpnTest in http://aosp/2624812
Change-Id: Ib9baa40d6dc870a548ebf8332f2829f4e49be428