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/bp2build/testing.go b/bp2build/testing.go
index 53b60fa..156e082 100644
--- a/bp2build/testing.go
+++ b/bp2build/testing.go
@@ -25,14 +25,17 @@
 	"testing"
 
 	"android/soong/android"
+	"android/soong/android/allowlists"
 	"android/soong/bazel"
 )
 
 var (
 	// A default configuration for tests to not have to specify bp2build_available on top level targets.
-	bp2buildConfig = android.Bp2BuildConfig{
-		android.BP2BUILD_TOPLEVEL: android.Bp2BuildDefaultTrueRecursively,
-	}
+	bp2buildConfig = android.NewBp2BuildAllowlist().SetDefaultConfig(
+		allowlists.Bp2BuildConfig{
+			android.Bp2BuildTopLevel: allowlists.Bp2BuildDefaultTrueRecursively,
+		},
+	)
 
 	buildDir string
 )