VPN reconnection fails after manually disabling VPN

When disabling VPN manually, there was no trigger for ipsec-tools to
send “delete message” to VPN server. Therefore, connection information
is left in VPN server and next connection fails.

Fix this issue as below:
- Add “delete message” sending via flush in ipsec-tools when racoon
  daemon stops
- Keep daemon alive when VPN.java exit() to let it finish sending to
  VPN server
- Move close(socket) and stop(daemon) in VPN.java execute()
  and monitorDaemons() to run() to gather cleaning block.

Bug: 28279646
Change-Id: Ie5a9497189da8c96321a220ea21ebe63e42d0751