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/bazel/Android.bp b/bazel/Android.bp
index 80af2bd..9e7edc7 100644
--- a/bazel/Android.bp
+++ b/bazel/Android.bp
@@ -10,11 +10,11 @@
         "configurability.go",
         "constants.go",
         "properties.go",
+        "testing.go",
     ],
     testSrcs: [
         "aquery_test.go",
         "properties_test.go",
-        "testing.go",
     ],
     pluginFor: [
         "soong_build",