How to enable an Instant App in Unity without Google's SDK?

I read that to enable an Android build as an Instant App, you need to add android:targetSandboxVersion="2" to the AndroidManifest.xml.

I added this to Assets/Plugins/Android/AndroidManifest.xml, but it is not getting included in my final APK:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android" android:targetSandboxVersion="2">
  <application>
  </application>
</manifest>

Or perhaps it's possible to achieve this with Gradle?

I don't want to use Google's Instant Play Unity SDK, I am trying to do it manually.

Any help is appreciated :)