Create (API) bp2build converters for droidstubs

- The converter runs for api_bp2build, not bp2build workspace
- Since droidstubs is an internal module created by java_sdk_library,
  the conversion encompasses the latter as well
- Since droidstubs do not have an api_surface attribute, this conversion
  uses naming convention to infer the api_surface represented by the api
  file e.g. *stubs.source -> publicapi, *stubs.source.system -> systemapi)
- Also adds an SdkIntraCore enum to represent the API surface provided
  by one core module to another

There is also ongoing work to check in java_api_contribution modules in
Soong. Once we have that, we can update this converter to operate on
that module type instead

Test: go test ./bp2build
Change-Id: Ia85828e04c738d9ffcc524856d7c3034ee29bbf9
diff --git a/android/sdk_version.go b/android/sdk_version.go
index c188c48..d73c912 100644
--- a/android/sdk_version.go
+++ b/android/sdk_version.go
@@ -44,6 +44,7 @@
 	SdkNone
 	SdkCore
 	SdkCorePlatform
+	SdkIntraCore // API surface provided by one core module to another
 	SdkPublic
 	SdkSystem
 	SdkTest
@@ -69,6 +70,8 @@
 		return "core"
 	case SdkCorePlatform:
 		return "core_platform"
+	case SdkIntraCore:
+		return "intracore"
 	case SdkModule:
 		return "module-lib"
 	case SdkSystemServer: