libcutils: fs_config() add check /vendor/etc/fs_config_*

Add reading of vendor file-system config files
/vendor/etc/fs_config_dirs and /vendor/etc/fs_config_files.

Order of interpretation (for dirs and files respectively):
- /system/etc/fs_config_dirs or /system/etc/fs_config_files
- /vendor/etc/fs_config_dirs or /vendor/etc/fs_config_files
- internal android_dirs[] or android_files[] structures.

No restrictions are placed on the vendor file-system config files,
although the developer is advised to restrict the scope to the /vendor
file-system since the intent is to provide support only for
customized portions of vendor.img.

Test: full build and install smoke test and inspection
Bug: 36071012
Change-Id: I4077bd6afcda2ee16189b2eb3c322af15205bbb9