[Application] PackageInstaller doesn't need to be RefCounted

That capability is not used in the code, so it's better to remove it. An
unused constructor was also removed.

This patch also adds a small class comment to PackageInstaller.
[Application] Move handling of management commands to ApplicationSystem

This patch is more about moving this logic out of XWalkBrowserMainParts
to a more reasonable location. We may decide to move this handling code
later (to hid it more) when implementing the IPC-based handling of the
mode.

Another outcome is that the logic for launching applications (and all
its cases) is clearer now.
[Application] Move logic for launching app to ApplicationSystem

Main motivation was to abstract this complexity away from
XWalkBrowserMainParts. ApplicationSystem now deals with the different
ways to pass the app id in the command line.

It turns out that after the refactoring, we can see that Tizen sensor
initialization wasn't being called in some cases when running
applications.
[Application] Remove unused includes from XWalkBrowserMainParts