Use metadata_key_dir instead of metadata_encryption

Eric pointed that metadata_key_dir is what specifies that metadata encryption
is enabled, whereas metadata_encryption is the metadata encryption options,
which don't have to be specified if the default options are sufficient.

Bug: 208161227
Fixes: 80a1d8e3 ("Avoid to format raw disk if metadata encryption is enabled")
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: I390450ba49c528b1f465c247d32174d36c7937a2