build: Split out mkfs in BuildImageMkfs (retry)

NB: second attempt, issue with scope for mkfs_output.

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 thus do
so without verity or avb wrappings.  Add partition_headroom to space
consideration.  This makes the results of the estimation more
accurate and predictable.

Test: build
Bug: 111302946
Change-Id: I2549bd4e403c21290470b2fa1835492ae883f0fd