Display ongoing app-installation at top of the list

Currently, in the "Installed" page, the app whose installation is in progress and the apps which are queued for installation (marked as "Pending") are given the same rank while sorting. In this patch, we change the priority, giving a higher priority to the current installing app so that it shows at the top, followed by the "Pending" apps for installation.

This is a partial fix for: https://phabricator.endlessm.com/T23263