Switch bp2build mutator to bottom up

This should be no-op, as the underlying mutator has not changed yet.

Some other refactoring is required and done in this CL:

- Delete some old, dead ApiBp2build code
- Fix casting to TopDownMutator when it's not necessary

This change is required to prepare for allowlist v2 work, as only
BottomUp mutators can AddDependency.

Bug: 285631638
Test: m nothing
Test: presubmits
Change-Id: I5212a5f5634cc13056195783e6df37ff8eb000da