More volume UUID awareness.

Teach free_cache() and restorecon_data() about building per-volume
paths.  Also clean up restorecon_data() by using std::string when
building paths.

Clearer names for path building utility methods, and tests to verify.

Bug: 19993667
Change-Id: Iacfbcdaa5b901cc2490bc8eba366dfdeb44f1d93