[Application] Add Application API context, launch, kill support

The supported APIs are listed below:
  ApplicationManager.getCurrentApplication(),
  ApplicationManager.kill(),
  ApplicationManager.launch(),
  and Application, ApplicationContext intefaces.

The application launch/kill operations will be implemented in xawlk-launcher and triggered by DBus client in the DBus agent.

The application context ID is a running Tizen app's PID which can be retrieved from AMD(Application Management Daemon implemented in AUL package). We need to fetch such info from xwalk-launcher too.

The DBus agent will be created for DBus communication between xwalk runtime and launcher. The instance of this class will be owned by ApplicationManager class.