Add PPTP, L2TP/IPSec preshared key and other fixes.

* Changes
  + Add PptpProfile.java, PptpService.java.
  + Add L2tpIpsecPskProfile.java
  + Add PPTP and L2TP_IPSEC_PSK entries in VpnType.java.
  + Add the secret option to L2tpProfile.java and have L2tpIpsecProfile
    extend it.
  + Add MtpdHelper to send common PPP options.
  + Add getGatewayIp() to VpnService().
  + Revise VpnService.reallyGetHostIp().

  Patch Set 6, 7, 8:
  + Add L2TP secret flag in L2tpProfile.

  Patch Set 9:
  + Add description to VpnType.

  Patch Set 11:
  + Pass ipparam to mtpd.

  Patch Set 12:
  + Add L2TP secret support
  + Fix string constants