Remove VPN Overflow menu

- Move always-on option for legacy vpn into the legacy vpn config page
- This implementation doesn't show dialogue when replacing existing always-on vpn
- Continue to disable lockdown option for legacy vpn when "persist.radio.imsregrequired" is true.
  Not applying to vpn app
- Force to save account info when legacy vpn is always-on
- When legacy vpn is always-on, don't try to connect. (Otherwise, an exception is thrown)

TODO: Remove EXTRA_PICK_LOCKDOWN in LockdownVpnTracker in framework

Bug: 26950700
Change-Id: Ia80669359c0b7cdb955c84937156c020ac6e9af5