Fail with an error if we can't read the policy for encryption

The absence of a policy reference in the unencrypted directory now
causes e4crypt_set_directory_policy to fail with an error. Callers
should call e4crypt_is_native (now moved into here) before calling this.

Bug: 28318405
Change-Id: I209292aba3abad3b19105c9afe2b84e8b3dd6874
3 files changed