Revise VpnService to use new vpn.* properties.

* Changes
  + Make VpnService get dns info from vpn.dns1/dns2 and set
    net.dns1/dns2.
  + Make VpnService wait on vpn.up instead of monitoring on ip-up-vpn.
  + Change commands sent to racoon.
  + Remove original sendCommand() from AndroidServiceProxy and rename
    sendCommand2() to sendCommand().
  + Add L2tpIpsecPskService.java.
  + Add short stop after sending ipsec command.