[Multi-user] Route USER_ALL calls to USER_SYSTEM for restoreAtInstall

Before ag/5882734 where we introduced the user param for
restoreAtInstall(), PackageManager would route all calls to the system
user including USER_ALL. Continue to route USER_ALL to USER_SYSTEM as
CTS infrastructure installs packages with USER_ALL.

Bug: 122954632
Test: atest CtsBackupHostTestCases
Change-Id: Icb674435f70adcd4b0349f13acc1ff83185575ac