Generate java_api_contribution module from droidstubs module

Context
- droidstubs module are either generated from the java_sdk_library
  module or defined in the bp files.
- Since droidstubs module contains API text file property,
  java_api_contribution module can by dynamically created from
  droidstubs.

Implementation
- Add `api_surface` property in droidstubs module. This property is
  either inherited from the java_sdk_library or written in the module
  definition in the bp file.
- Add defaultable hook in droidstubs module to generate the child
  java_api_contribution module.

Test: m
Change-Id: Ica43d65614723c623cd0c155266f9844e69e5d5e

java/sdk_library.go

@@ -1599,6 +1599,7 @@ func (module *SdkLibrary) createStubsSourcesAndApi(mctx android.DefaultableHookC
 		Srcs                             []string
 		Installable                      *bool
 		Sdk_version                      *string
+		Api_surface                      *string
 		System_modules                   *string
 		Libs                             []string
 		Output_javadoc_comments          *bool
@@ -1638,6 +1639,7 @@ func (module *SdkLibrary) createStubsSourcesAndApi(mctx android.DefaultableHookC
 	props.Srcs = append(props.Srcs, module.properties.Srcs...)
 	props.Srcs = append(props.Srcs, module.sdkLibraryProperties.Api_srcs...)
 	props.Sdk_version = module.deviceProperties.Sdk_version
+	props.Api_surface = &apiScope.name
 	props.System_modules = module.deviceProperties.System_modules
 	props.Installable = proptools.BoolPtr(false)
 	// A droiddoc module has only one Libs property and doesn't distinguish between