cryptohome: Test for symlinks before copying attestation file. BUG=chromium:649039 TEST=Rebooted device with normal file vs. symlink. Change-Id: I3a86d5bec4495f8a0dd9e038ab3278bd394517d8 Reviewed-on: https://chromium-review.googlesource.com/388186 Commit-Ready: Ricky Zhou <[email protected]> Tested-by: Ricky Zhou <[email protected]> Reviewed-by: Mike Frysinger <[email protected]> Reviewed-on: https://chromium-review.googlesource.com/389651 Reviewed-by: Ricky Zhou <[email protected]> Commit-Queue: Ricky Zhou <[email protected]>