fs_mgr: allow specifying HEH filenames encryption

Make the fs_mgr changes needed to support specifying aes-256-heh
filenames encryption.  To do this, extend the "fileencryption" fstab
option to have the syntax
fileencryption=contents_encryption_mode[:filenames_encryption_mode].
The previous filenames encryption mode, aes-256-cts, remains supported
and will be used by default if no mode is explicitly specified.

For consistency, also make "aes-256-xts" an alias for the "software"
contents encryption mode.

Bug: 34712722
Change-Id: I74c0f5fe7cddde7055d7b031d96a2916ca611eb7