Avoid extra copy of JNI libs zip

Don't call merge_zips if there are no prebuilt JNI libraries to
merge with.

Test: builds
Change-Id: Ife5386fbbfcff35bdea2914519a78dfd89163a25
diff --git a/java/app_builder.go b/java/app_builder.go
index d20a6bf..e241adb 100644
--- a/java/app_builder.go
+++ b/java/app_builder.go
@@ -225,8 +225,6 @@
 	})
 }
 
-const jniJarOutputPathString = "jniJarOutput.zip"
-
 func TransformJniLibsToJar(
 	ctx android.ModuleContext,
 	outputFile android.WritablePath,
@@ -258,7 +256,10 @@
 		rule = zipRE
 		args["implicits"] = strings.Join(deps.Strings(), ",")
 	}
-	jniJarPath := android.PathForModuleOut(ctx, jniJarOutputPathString)
+	var jniJarPath android.WritablePath = android.PathForModuleOut(ctx, "jniJarOutput.zip")
+	if len(prebuiltJniPackages) == 0 {
+		jniJarPath = outputFile
+	}
 	ctx.Build(pctx, android.BuildParams{
 		Rule:        rule,
 		Description: "zip jni libs",
@@ -266,12 +267,14 @@
 		Implicits:   deps,
 		Args:        args,
 	})
-	ctx.Build(pctx, android.BuildParams{
-		Rule:        mergeAssetsRule,
-		Description: "merge prebuilt JNI packages",
-		Inputs:      append(prebuiltJniPackages, jniJarPath),
-		Output:      outputFile,
-	})
+	if len(prebuiltJniPackages) > 0 {
+		ctx.Build(pctx, android.BuildParams{
+			Rule:        mergeAssetsRule,
+			Description: "merge prebuilt JNI packages",
+			Inputs:      append(prebuiltJniPackages, jniJarPath),
+			Output:      outputFile,
+		})
+	}
 }
 
 func (a *AndroidApp) generateJavaUsedByApex(ctx android.ModuleContext) {