Add check_builtins to libinit_host

This requires a bit of refactoring: moving things around.

libinit_host is used by host_apex_verifier which needs check_builtins as
well.

Bug: 325565247
Test: atest host-apex-verifier
Test: m out/target/product/vsoc_x86_64/host_init_verifier_output.txt
Change-Id: Ifed54dd2149afbab2bf63f7e42c410c2354895fc