Android: enable auto-connect feature, always-on

[alalamav]

• Adds support for Always On VPN.
• Enables the auto-connect feature: automatically connects the VPN, unconditionally if the user has enabled Always On, or if the VPN was connected at shutdown. The latter behavior targets Lollipop and Marshmallow users, as Always On was introduced in Nougat.
• Does not perform connectivity checks when connecting at startup to avoid networking errors if the network is not ready.
• Persists the connection status to support both Always On and auto-connect - part of this code was already merged but not enabled.