Refactor how bp2build gets arch-specific props.

Then plumb them to LabelAttribute.

This refactoring is required because the previous implementation did not
handle properties in shards other than the first one (e.g.
version_script) well. In addition, it also makes the code paths between
bp2build and analysis more similar.

Bug: 186650430
Test: Presubmits.
Change-Id: Ic4393e8ae47f4e88816bf45c89399efd61494d22