fs_mgr_fstab: support specifying encryption policy version in fstab

Allow the encryption policy version to be specified as a third field in
the "fileencryption=" fstab option, and expose it through the fstab
record so that vold can use it.

Bug: 140500999
Test: tested as series; see If64028d8580584b2c33c614cabd5d6b93657f608.
      Also updated and ran the fs_mgr unit tests.
Change-Id: If5d56f4c84c4cd51e3f14c0a861c3e3ab6ff903f