Bug 964653 - (system-dialog) [Window Management] Implement System Dialog Window

Since some bad changesets to the Gaia repo, the changed of aa48f672f07fe18c590d31d6d19147193c0269ad  is removed without revert commit. So, I file another pr for landing it again. Cherry pick from 022ec711569e0c49ceebbe27039b38722ff34ad5, this patch is fixed unit test failed which cause from conflict in mock module.