copy JNI from AAR files to android_app APK

JNI libs present in AAR files are ignored in the build. If those
libraries are necessary, they must be manually extracted and a
prebuilt_cc_library module must be created for them. Instead, we can
extract these libraries and copy them to an output APK as part of the build.

Bug: 228848794
Test: created android_app that depends on an android_library_import with
  JNI embedded, and verified that .so libs are present in APK
Test: created android_app that depends on an android_library which
  depends on android_library_import with JNI embedded, and verified
  that .so libs are present in APK
Test: verified that multiple .so's from a single architecture are copied
  to APK
Test: verified that the absence of a jni/<arch> folder or files in the
  folder will cause the Ninja action to error
Change-Id: I242a862d7bd881f7a4a8c23493924d8fe441ea25
diff --git a/java/app_builder.go b/java/app_builder.go
index 4a18dca..4348644 100644
--- a/java/app_builder.go
+++ b/java/app_builder.go
@@ -218,8 +218,14 @@
 	})
 }
 
-func TransformJniLibsToJar(ctx android.ModuleContext, outputFile android.WritablePath,
-	jniLibs []jniLib, uncompressJNI bool) {
+const jniJarOutputPathString = "jniJarOutput.zip"
+
+func TransformJniLibsToJar(
+	ctx android.ModuleContext,
+	outputFile android.WritablePath,
+	jniLibs []jniLib,
+	prebuiltJniPackages android.Paths,
+	uncompressJNI bool) {
 
 	var deps android.Paths
 	jarArgs := []string{
@@ -245,13 +251,20 @@
 		rule = zipRE
 		args["implicits"] = strings.Join(deps.Strings(), ",")
 	}
+	jniJarPath := android.PathForModuleOut(ctx, jniJarOutputPathString)
 	ctx.Build(pctx, android.BuildParams{
 		Rule:        rule,
 		Description: "zip jni libs",
-		Output:      outputFile,
+		Output:      jniJarPath,
 		Implicits:   deps,
 		Args:        args,
 	})
+	ctx.Build(pctx, android.BuildParams{
+		Rule:        mergeAssetsRule,
+		Description: "merge prebuilt JNI packages",
+		Inputs:      append(prebuiltJniPackages, jniJarPath),
+		Output:      outputFile,
+	})
 }
 
 func targetToJniDir(target android.Target) string {