Apex interface now using FileSystem

Apex relying on real IO caused special handling when creating
VintfObject especially on host. (Needed to `resolve()` first to create
Apex).

Now Apex performs IO using FileSystem. And using Apex is guarded by
property "apex.all.ready=true" via PropertyFetcher.

Bug: 266783080
Test: libvintf_test vintf_object_test
Change-Id: Ibf572e9240e841f0c62bbd817fb845bc6230ff76
11 files changed