[pm] set loading progress to 100% for system apps

Before, we only assign loading progress to new installs, which resulted
in empty progress ring hovering around system apps. This CL assigns
loading progress to all the installs including the system apps.

BUG: 177367713
Test: manual with Google Phones install updates and uninstall updates
Change-Id: Ie83c263fefc324ad430b15694b3321740f2fe332