Rename FS_MGR_FS_STAT into FS_MGR_FS_STAT_DATA_PARTITION

- FS_STAT can be returned per each partition mounted. Currently only
  /data partition is checked but rename it so that we can add additional
  partitions in future.

Bug: 132691841
Test: Build
Change-Id: I3f1280ce84df1137a92ac59b15937a32c7ac8d01