Changes UserSystemPackageInstaller log TAG to use its own class name.

This class issues a lot of "PKG is allowlisted but not present" logs
on boot, which makes it harder to watch other log messages on
UserManagerService.

Test: adb logcat UserManagerService *:s
Bug: 226643927

Change-Id: I8dbd32fd06b133b7e7136d11133a2fac121d1b9b