Make generateFsverityMetadata return on-disk data

It used to return fs-verity measurement, but we need the on-disk format
for setup.  The on-disk format just has extra algorithm ID and digest
length.

Test: Observe fs-verity setup happens, with backported fs-verity to Pixel 3
Bug: 112037636
Change-Id: I446656e42c05dad1ffa272c3e3d623006490b1cc