fs_mgr_fstab: Add Adiantum support

Adiantum is a crypto method Android is supporting for devices
which don't have AES CPU instructions.  See the paper
"Adiantum: length-preserving encryption for entry-level processors"
(https://eprint.iacr.org/2018/720.pdf) for more details.

We add Adiantum to our list of supported encryption modes.

Bug: 112010205
Test: Tested on a device
Change-Id: I14a400164803a1e217d378ad9bd8b67a61b8b7d0