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/Android.bp b/android/Android.bp
index 49d5b91..87b021f 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -19,6 +19,7 @@
         "soong-shared",
         "soong-starlark-format",
         "soong-ui-metrics_proto",
+        "soong-android-allowlists",
 
         "golang-protobuf-proto",
         "golang-protobuf-encoding-prototext",