fs_mgr: make fs_mgr_setup_verity public API

Consequently this removes the 'early' version of the exact same API.
This is to be used by early mount code where we need to do
1. setup verity
2. create verity device
3. mount partition
as separate steps since that happens during init first stage.

b/27805372

Test: Boot sailfish successfully

Change-Id: I01abecfdfa210d3e240a291ddcb3d2e9ed39ede6
Signed-off-by: Sandeep Patil <sspatil@google.com>