Cleanup global FileFetcher
This global instance is modifiable everywhere and
is hard to maintain. It also contains references as
out-variables, which is not the standard.
Clean it up by refactoring it into a public (i.e.
not in details namespace) class, FileSystem.
Introduce VintfObject::InitFileSystem that allows
the behavior to be changed before any files are read.
Test: host and target libvintf_test, vintf_object_test
12 files changed