Duplicate fs-verity struct and constants if no header

This is a hack to make fs-verity work against the current proposal.
Once fs-verity is upstreamed and kernel header updated in bionic,
we should remove the hacks.

Test: with fsverity.h and without, and feature flag enabled run
      `atest com.android.cts.apkverity.ApkVerityTest` (new)
Bug: 111316641

Change-Id: I74d639a82a0e2adbb1efe0cce0aa5c861b858e97