Have ConvertWBp2build use Bp2buildMutatorContext

This no-op refactoring facilitates some upcoming functional changes for
"bp2build allowlist v2". The work requires that the bp2build conversion
mutator be changed from a TopDown mutator to a BottomUp mutator.
Refactoring all bp2build-related methods so that they use Bp2buildMutatorContext
makes it easier to make this functional change without touching tens of
files and multiple projects.

Bug: 285631638
Test: m bp2build
Change-Id: I3d1ef3064146e959c6f0dc315350fc9764bf2bd2
diff --git a/rust/protobuf.go b/rust/protobuf.go
index ae82844..c8d2bda 100644
--- a/rust/protobuf.go
+++ b/rust/protobuf.go
@@ -282,7 +282,7 @@
 	Srcs bazel.LabelListAttribute
 }
 
-func protoLibraryBp2build(ctx android.TopDownMutatorContext, m *Module) {
+func protoLibraryBp2build(ctx android.Bp2buildMutatorContext, m *Module) {
 	var protoFiles []string
 
 	for _, propsInterface := range m.sourceProvider.SourceProviderProps() {