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
- Since daemon will stop itself when monitor socket is closed, replace
  stop(daemon) with close(socket); thus remove redundant socket closing
  in VPN exit()

(cherry picked from commit cb1e9555)

Bug: 28279646
Bug: 33467086
Change-Id: I64591874a1de5d0dd4b5834211d07effb962cd6b