Split xwalkctl and pkgcmd backend

All the application management (app un/installation) logic is now removed
from it 'xwalkctl'- it is only responsible for XWalk common settings,
like setting of debugging port.

The application management is moved to a separate target: xwalk_backend.
The wrapping script is removed as well.

BUG=XWALK-2671
BUG=XWALK-2579