Don't set sehash when calling restoreconRecursive

restorecon_recursive updates the SELinux label of the files in the
filesystem, and then attempts to write the xattr "security.sehash" as an
optimization for future restorecons. Writing security.* extended
attributes requires CAP_SYS_ADMIN, which system_server doesn't have (and
shouldn't have).

Suppress the computation and writing of the hash value. It's not
needed.

This bug has been around for a long time, but due to the fix for
bug 62302954, the error message is being generated more frequently
now.

TODO: It would be better if the default for restorecon was to suppress
the hash computation, since otherwise it encourages programs to be
overprivileged with CAP_SYS_ADMIN. I'll plan on doing that in a followup
commit.

Bugs where this error message has been called out:
Bug: 129766333
Bug: 129271240
Bug: 128700692
Bug: 129925723

Test: install an APK and ensure that no "SELinux:  setxattr failed"
    error messages are generated.
Change-Id: I83f4f225e4e73453daea42dbcabf0f8a06d320d6