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
diff --git a/cc/proto.go b/cc/proto.go
index 0ed4381..4e08a4c 100644
--- a/cc/proto.go
+++ b/cc/proto.go
@@ -246,7 +246,7 @@
 	protoAttrs.Min_sdk_version = m.Properties.Min_sdk_version
 
 	name := m.Name() + suffix
-	tags := android.ApexAvailableTagsWithoutTestApexes(ctx.(android.TopDownMutatorContext), m)
+	tags := android.ApexAvailableTagsWithoutTestApexes(ctx, m)
 	ctx.CreateBazelTargetModule(
 		bazel.BazelTargetModuleProperties{
 			Rule_class:        rule_class,