Export stub sources as part of the java_sdk_library

Minor refactoring of checkMergeZip(string) -> checkMergeZips(...string)
to allow testing of multiple merge zips.

Bug: 153443117
Test: m nothing
Change-Id: I8db00f611ced15f8476ba16f2834a72e8c913596
diff --git a/java/droiddoc.go b/java/droiddoc.go
index b0efaa5..6c3c7ec 100644
--- a/java/droiddoc.go
+++ b/java/droiddoc.go
@@ -370,10 +370,17 @@
 	apiToCheck.Removed_api_file = nil
 }
 
+// Used by xsd_config
 type ApiFilePath interface {
 	ApiFilePath() android.Path
 }
 
+// Provider of information about API stubs, used by java_sdk_library.
+type ApiStubsProvider interface {
+	ApiFilePath
+	StubsSrcJar() android.Path
+}
+
 //
 // Javadoc
 //
@@ -1259,6 +1266,10 @@
 	return d.apiFilePath
 }
 
+func (d *Droidstubs) StubsSrcJar() android.Path {
+	return d.stubsSrcJar
+}
+
 func (d *Droidstubs) DepsMutator(ctx android.BottomUpMutatorContext) {
 	d.Javadoc.addDeps(ctx)