Implement bp2build for the `license` module

Also, add ExpectedRuleTarget type to simplify bp2build testing

Bug: 190817312
Test: treehugger
Change-Id: Id3a642680d3518d36360ba957919a6fc96222672
diff --git a/bp2build/Android.bp b/bp2build/Android.bp
index 3d9fc5a..8aef66f 100644
--- a/bp2build/Android.bp
+++ b/bp2build/Android.bp
@@ -63,6 +63,7 @@
         "java_library_host_conversion_test.go",
         "java_plugin_conversion_test.go",
         "java_proto_conversion_test.go",
+        "license_conversion_test.go",
         "linker_config_conversion_test.go",
         "ndk_headers_conversion_test.go",
         "performance_test.go",