Fixed testCreateAndManageUser_LeaveAllSystemApps() for headless system user.

It was failing because some system apps that should not be
uninstalled were not available in the new user, but they were not
available because of the allowlist mechanism introduce on R: these
apps were just installed on SYSTEM users, not on FULL users (which
is the type of the managed user created by the test).

Bug: 179905667
Fixes: 181059137

Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_LeaveAllSystemApps # on automotive and phone

Change-Id: Ie44124737a4a20523b6a79ecbae9049cdacd393e