libcutils: fs_config read from file

Intention is to read from ${OUT}/system/etc/fs_config_dirs and
${OUT}/system/etc/fs_config_files on host. And
/system/etc/fs_config_dirs and /system/etc/config_files
on target systems.

Bug: 19908228
Change-Id: I0966a94f79a3fae4f7325056c701ea355370f9df