Use "system" background category

Use "system" background category

This patch solves issue with terminating oobe app during system boot.
The app was sometimes terminated by AMD daemon because
it managed to launch faster then homescreen app.

When such situation occured, the AMD daemon had internal logic that
do not allow apps that are not homescreen and are not task-managable to
run as app leader.

To solve this issue the additional special flag was added to tell AMD to ignore
this case.

Change-Id: I4ec39e9bb168a97b1351a57237180a5f86d7860c