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

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