tunnel: stop vpn adapter when another app takes over the vpn

[SaadAhmedGit]

fixes: #1545

This small change fixes the issue mentioned in the referenced issue. When another app takes over the VPN, onRevoke() method of the VpnService() is called which wasn't overriden to call stopVpnAdapter() which closes the tunnel.

Overriding onRevoke() to call stopVpnAdapter() ensures the tunnel is closed gracefully when another app starts the VPN.