The callback of stop previous VPN session may happen after new session started when starting a new vpn session

[enuoCM]

Test on latest commit d634606.
I added some log messages in OpenVPNService#stopOldOpenVPNProcess(), OpenVPNService#forceStopOpenVpnProcess, OpenVPNService#processDied() and OpenVPNService#unregisterDeviceStateReceiver().
Sometimes I got following log:

06-26 11:24:19.052 D/test   (25945): stopOldOpenVPNProcess mManagement: com.cloudminds.security.mimclient.openvpn.core.OpenVpnManagementThread@eb820af
06-26 11:24:20.083 D/test   (25945): forceStopOpenVpnProcess: interrupt the old mProcessThread: Thread[MIMOpenVPNProcessThread,5,main]
06-26 11:24:21.084 D/test   (25945): a new mProcessThread created for the new vpn session, mProcessThread: Thread[MIMOpenVPNProcessThread,5,main]
06-26 11:24:21.085 D/test   (25945): unregisterDeviceStateReceiver!!! (happend in startOpenVPN# new Handler(getMainLooper()).post)
06-26 11:24:21.086 D/test   (25945): registerDeviceStateReceiver: mDeviceStateReceiver:com.cloudminds.security.mimclient.openvpn.core.DeviceStateReceiver@3ca6c79a (happend in startOpenVPN# new Handler(getMainLooper()).post)
06-26 11:24:21.157 D/test   (25945): processDied: mDeviceStateReceiver:com.cloudminds.security.mimclient.openvpn.core.DeviceStateReceiver@3ca6c79a
06-26 11:24:21.158 D/test   (25945): unregisterDeviceStateReceiver!!! (happend in processDied()->endVpnService())

You can find that the new regiestered DeviceStateReceiver@3ca6c79a was removed incorrectly. In fact more incorrect actions were done in processDied() under this case. I think here is a synchronizion issue.