fs_mgr: add libfstab for vendor

libfstab is a subset of libfs_mgr, intended for vendors to use. It
exposes APIs for reading fstab. Note this 'visible to vendor' does not
mean that the API should be stable forever. The API can be changed in
later releases of Android, ,but the newer Android must not cause
run-time error when there is an older version of this static library
being used somewhere.

Bug: 62990533
Test: BOARD_VNDK_VERSION=current m -j libfstab.vendor
Change-Id: If8fc73e4ae4c2f8281c41a12f1c18079aab8baa2