Don't require private types in mapping file.

Private types are not visible to vendor/odm policy, so we don't need mapping
entries for them.

We build platform-only public policy .cil file and give it as input to
treble_sepolicy_tests. Using this public policy the test can now figure out if
the newly added type in public or private.

Bug: 116344577
Test: adding public type triggers mapping test failure, adding private type does
not.
Change-Id: I421f335e37274b24aa73109e260653d7b73788b5