[Multi-user] Properly check for system apps in multi-user context

The previous check compared app uid directly to FIRST_APPLICATION_UID
which doesn't work for non-system users as uid incorporates the user id
(so system apps in non-system users would always be categorized as
non-system apps).

Bug: 122309666
Test: 1) atest RunBackupFrameworksServicesRoboTests
2) Manual: verify system packages backed up for system user, not backed
up for non-system user.

Change-Id: I5e11b4b7c41d74d43d545e4831d09a9a3a59e5cf