refactor bp2build module allowlists into struct

To make testing easier, refactor existing module-global variables into a
struct that can be mocked.

Test: build/bazel/bp2build.go
Change-Id: I9d177677644ea743641a745b1839a3a8b29f902a
diff --git a/android/testing.go b/android/testing.go
index a9632e9..ac02db9 100644
--- a/android/testing.go
+++ b/android/testing.go
@@ -211,7 +211,7 @@
 	ctx.finalDeps = append(ctx.finalDeps, f)
 }
 
-func (ctx *TestContext) RegisterBp2BuildConfig(config Bp2BuildConfig) {
+func (ctx *TestContext) RegisterBp2BuildConfig(config bp2BuildConversionAllowlist) {
 	ctx.config.bp2buildPackageConfig = config
 }