build: Split out mkfs in BuildImageMkfs

Separate out BuildImageMkfs from BuildImage, which just makes the
filesystem without any verity, avb, or other decisions.  BuildImage
does all the wrapping for such.  This will hopefully ease maintenance
and drop the issues surrounding BuildImage reentrancy.  Change
right-size estimation path to use BuildImageMkfs, and do so without
verity or avb wrappings.

Test: build
Bug: 111302946
Change-Id: I30e2e2b727f40ecca5164142f34139f5244f6424