[connectivity_plus] Refactor the C++ code

[swift-kim]

Part of the code refactoring project. This change is an analogue of #351.

• Extract the Connection class from ConnectivityPlusTizenPlugin for better separation of the platform channel communication and the device API usage. (I referred to the connectivity_plus_windows implementation.)
  • Create an enum ConnectionType to indicate the current connectivity type or an error status.
  • RegisterObserver/ClearObserver → StartListen/StopListen
  • StartListen takes ConnectionTypeCallback as an argument.
  • Create a new method GetType.
  • Remove EnsureConnectionHandle. The connection handle is now created and managed by the Connection class.
• Create typedefs of long commonly-used types.
• Create an explicit ConnectivityStreamHandler class. Now the lifecycle of EventSink is solely managed by this class.
• Minor cleanups.
  • Remove SetupChannels and combine with RegisterWithRegistrar.
  • Style log messages and error codes in a consistent manner.
  • Wrap the non-public implementation with an anonymous namespace.

Additional note: The connectivity type "bluetooth" has been added to the platform interface and will be implemented in a later PR.