init/fscrypt_init_extensions: support setting v2 encryption policies

Support setting v2 encryption policies on init-created directories.  The
policy version to set is gotten from a new field in
/data/unencrypted/mode, which is the file that's used to pass the
encryption options from vold to init.

Also don't bother falling back to defaults if fields are missing from
this file, since it's re-written on every boot by vold.

Bug: 140500999
Test: tested as series; see If64028d8580584b2c33c614cabd5d6b93657f608
Change-Id: Ia9c5d4b80199686799e3ac80de78a50ed3bdabf4