Add support for .proto files in java modules

Test: m -j checkbuild
Change-Id: Ia03429948baebff85164a91a34507866c97a08ef
diff --git a/java/builder.go b/java/builder.go
index 95345d4..b924d65 100644
--- a/java/builder.go
+++ b/java/builder.go
@@ -127,6 +127,9 @@
 	desugarFlags  string
 	aidlFlags     string
 	javaVersion   string
+
+	protoFlags   string
+	protoOutFlag string
 }
 
 func TransformJavaToClasses(ctx android.ModuleContext, srcFiles, srcFileLists android.Paths,
@@ -136,7 +139,10 @@
 	annoDir := android.PathForModuleOut(ctx, "anno")
 	classJar := android.PathForModuleOut(ctx, "classes-compiled.jar")
 
-	javacFlags := flags.javacFlags + android.JoinWithPrefix(srcFileLists.Strings(), "@")
+	javacFlags := flags.javacFlags
+	if len(srcFileLists) > 0 {
+		javacFlags += " " + android.JoinWithPrefix(srcFileLists.Strings(), "@")
+	}
 
 	deps = append(deps, srcFileLists...)
 	deps = append(deps, flags.bootClasspath...)
@@ -161,8 +167,8 @@
 	return classJar
 }
 
-func RunErrorProne(ctx android.ModuleContext, srcFiles android.Paths, srcFileLists android.Paths,
-	flags javaBuilderFlags, deps android.Paths) android.Path {
+func RunErrorProne(ctx android.ModuleContext, srcFiles, srcFileLists android.Paths,
+	flags javaBuilderFlags) android.Path {
 
 	if config.ErrorProneJar == "" {
 		ctx.ModuleErrorf("cannot build with Error Prone, missing external/error_prone?")
@@ -173,7 +179,12 @@
 	annoDir := android.PathForModuleOut(ctx, "anno-errorprone")
 	classFileList := android.PathForModuleOut(ctx, "classes-errorprone.list")
 
-	javacFlags := flags.javacFlags + android.JoinWithPrefix(srcFileLists.Strings(), "@")
+	javacFlags := flags.javacFlags
+	if len(srcFileLists) > 0 {
+		javacFlags += " " + android.JoinWithPrefix(srcFileLists.Strings(), "@")
+	}
+
+	var deps android.Paths
 
 	deps = append(deps, srcFileLists...)
 	deps = append(deps, flags.bootClasspath...)