Dedup apex-scanning

create_apex_data_dirs() now uses GetApexListFrom(dir) in
apex_init_util.cpp.

This is essentially a refactoring, but there are a few behavioral
changes, which I think make more sense.
- Emits no error when opendir(/apex) fails.
- Emits errors when mkdir fails for each apex.
- Does not abort `perform_apex_config` even though create_apex_data_dirs
  fails.

Bug: 293546778
Test: check /data/misc/apexdata/ after boot
Change-Id: I9d1a9710a6a626eec087c8c0fb1768667ffb036e