Fix that fail to setup any vpn after Network Settings reset and always-on vpn is on

Cause: It revoked the user consent of the vpn app without reseting always-on vpn.
In addition, prepareVpn sets legacy vpn as the current package, the state in
Vpn.class is broken, as it thought the current always-on package is legacy vpn,
(mAlwaysOn is only for app vpn, not for legacy vpn). As a result, prepareVpn rejects
all VpnService.prepare.

Bug: 29031820
Change-Id: Id6bf1d6f38cf134a872811806301b8a602fb5725