Make fsverity files able to be built independently

Right now, fsverity files are built when building the staging directory.
This is slow, before this cl it takes over a minute to generate the
staging directory, after it's only ~10 seconds.

Fsverity files are now generated before the staging directory is built,
in independant ninja actions.

Bug: 394404628
Test: m aosp_shared_system_image, verified it didn't change before/after

Change-Id: I9ebc40cd243888e3cb778487751937ccd79093bd