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/Android.bp b/bp2build/Android.bp
index 8a171d4..8f78968 100644
--- a/bp2build/Android.bp
+++ b/bp2build/Android.bp
@@ -18,6 +18,7 @@
     ],
     deps: [
         "soong-android",
+        "soong-android-allowlists",
         "soong-android-soongconfig",
         "soong-shared",
         "soong-apex",