bp2build: split Bazel conversion context into smaller ones,
and change TopDownMutatorContext signatures to use Bazel conversion context.

This minimizes the context interfaces/functions actually needed to
convert a module, and makes such interfaces easier to mock/test.

Test: CI
Change-Id: Id573d97023d59e06ef70e1f54437024d3f7aadbd
diff --git a/android/variable.go b/android/variable.go
index baa6dfa..e943640 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -498,7 +498,7 @@
 
 // ProductVariableProperties returns a ProductConfigProperties containing only the properties which
 // have been set for the module in the given context.
-func ProductVariableProperties(ctx BaseMutatorContext) ProductConfigProperties {
+func ProductVariableProperties(ctx BazelConversionPathContext) ProductConfigProperties {
 	module := ctx.Module()
 	moduleBase := module.base()