TestApi for UserSystemPackageInstaller

Introduces a TestApi that lists which system packages
will be pre-installed for the given user type, for
use with CTS tests that need to predict which packages
are installed on a new user.

Test: atest com.android.cts.devicepolicy.DeviceOwnerTest#testCreateAndManageUser_LeaveAllSystemApps
Bug: 189873486
Change-Id: I471c5da55f8cf500a07de5773050a997ea391892