fs_mgr: fix the fs_mgr_setup_verity param name

The 'verify_dev' parameter for fs_mgr_setup_verity is confusing, since
it doesn't necessarily control the "verification" part of the function.
Instead, it merely allows the caller to skip the
wait_for_dm_verity_device part of the function. So, rename that
parameter for what it is.

b/27805372

Test: Boot sailfish

Change-Id: I024e02fd728f5cd5bb333d9c3b512861731fa215
Signed-off-by: Sandeep Patil <sspatil@google.com>