How to sign android app with platform keys using gradle?

I saw couple similar questions like:

• Android: After building platform source, how to sign arbitrary APK with platform key?
• How to create a release signed apk file using Gradle?
• APK signing error : Failed to read key from keystore

but I feel my problem is different.

First of all I use:

android:sharedUserId="android.uid.system"

so I need to sign my app with platform key. I'm able to do that in this way:

cd $ANDROID_ROOT/out/host/linux-x86/framework
java -Djava.library.path=$ANDROID_ROOT/out/host/linux-x86/lib64 -jar signapk.jar $ANDROID_ROOT/build/target/product/security/platform.x509.pem $ANDROID_ROOT/build/target/product/security/platform.pk8 $APP_DIR/app/build/outputs/apk/debug/app-debug.apk $APP_DIR/MyApp-signed.apk

However I want to do signing from gradle, so I have generated jks file in this way:

./keytool-importkeypair -k my_keystore.jks -p my_password -pk8 $ANDROID_ROOT/build/target/product/security/platform.pk8 -cert $ANDROID_ROOT/build/target/product/security/platform.x509.pem -alias platform

and I've modified app/build.gradle to have:

 signingConfigs {
     release {
         storeFile file("my_keystore.jks")
         storePassword "my_password"
         keyAlias "platform"
         keyPassword "my_password"
     }
 }

 buildTypes {
     release {
         minifyEnabled false
         proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'

         signingConfig signingConfigs.release
     }
 }

I've checked that my_keystore.jks has platform alias with:

keytool -list -v -keystore my_keystore.jks | grep Alias
Alias name: platform

but when I try to do:

./gradlew assembleRelease

or:

./gradlew signingReport

I get:

Failed to read key platform from store "(...)/my_keystore.jks": Invalid keystore format

from How to sign android app with platform keys using gradle?