fs_mgr: Don't parse encryption options, just keep string

We now defer parsing encryption options to the fscrypt library. To avoid
adding a dependency, we simply record the options string in the fstab
and defer parsing until it's needed.

Bug: 143307095
Test: cuttlefish still boots
Change-Id: Ied13ea2f731c63b0524aed11db6983a86dab9fa1