Add android_app_import.

This is an initial version that handles the most basic cases.

Bug: 128610294
Test: app_test.go + prebuilt webview.apk
Change-Id: Ic525559aad5612987e50aa75b326b77b23acb716

java/app_builder.go

@@ -62,7 +62,7 @@ var combineApk = pctx.AndroidStaticRule("combineApk",
 		CommandDeps: []string{"${config.MergeZipsCmd}"},
 	})
 
-func CreateAppPackage(ctx android.ModuleContext, outputFile android.WritablePath,
+func CreateAndSignAppPackage(ctx android.ModuleContext, outputFile android.WritablePath,
 	packageFile, jniJarFile, dexJarFile android.Path, certificates []Certificate) {
 
 	unsignedApkName := strings.TrimSuffix(outputFile.Base(), ".apk") + "-unsigned.apk"
@@ -83,6 +83,11 @@ func CreateAppPackage(ctx android.ModuleContext, outputFile android.WritablePath
 		Output: unsignedApk,
 	})
 
+	SignAppPackage(ctx, outputFile, unsignedApk, certificates)
+}
+
+func SignAppPackage(ctx android.ModuleContext, signedApk android.WritablePath, unsignedApk android.Path, certificates []Certificate) {
+
 	var certificateArgs []string
 	var deps android.Paths
 	for _, c := range certificates {
@@ -93,7 +98,7 @@ func CreateAppPackage(ctx android.ModuleContext, outputFile android.WritablePath
 	ctx.Build(pctx, android.BuildParams{
 		Rule:        Signapk,
 		Description: "signapk",
-		Output:      outputFile,
+		Output:      signedApk,
 		Input:       unsignedApk,
 		Implicits:   deps,
 		Args: map[string]string{