Add the VPN services package, VPN service base classes and L2tpIpsecService.

Also add android.security.Keystore. This is a quick solution. Will be
evolved to a more mature implementation.

PATCH SET 2:
+ Add VpnServiceBinder to hide VpnService and its subclasses.
+ Add sendCommand2() to AndroidServiceProxy to work with the latest mtpd.

PATCH SET 3:
+ Rebase to catch up with new commits; no changes made.

PATCH SET 4:
+ Remove/comment out @Override on interface methods to be compilable for Java 1.5.

PATCH SET 5:
+ Add L2tpService.java.
+ Make VpnService to work on SingleServerProfile; add serverIp to
  connect(); set system property "net.vpn.server_ip"; and move
  getPppOptionFilePath() from L2tpIpsecService to VpnService
+ Revise VpnManager to start VpnService without worrying about which
  type (as the type info is in VpnProfile)
+ Remove installation stuff from VpnManager

PATCH SET 6:
+ Fix PATCH SET 5 (the patch was messed up). Please ignore PATCH SET 5.

PATCH SET 7:
+ Fix styles.

PATCH SET 8:
+ Add CANCELLED to VpnState.

PATCH SET 9:
+ Make VpnProfile serializable (in order to save them to persistent storage)

PATCH SET 10:
+ Remove Keystore.java as it's added in another CL.