add SELINUX_ANDROID_RESTORECON_SKIP_SEHASH

Allow callers to indicate that they don't want to compute the sehash
value. Callers may not have CAP_SYS_ADMIN, so attempting to write the
sehash value will result in the following (harmless) errors:

  SELinux:  setxattr failed: /data/app/com.andromeda.androbench2-z5oLVfPATqQF35yGDuMUeA==:  Operation not permitted

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.

(cherry picked from commit 3060b61d412fa30e91fb13dd29b82d994143aa30)

Change-Id: Iddfb221354ee6336c6ef99d7950b9d1a1a07d5ac
2 files changed