Update AndroidManifest.xml before build to set hardwareAccelerated to true

PreBuildManifestUpdate.cs

using System.IO;
using System.Text;
using System.Xml;
using UnityEditor.Android;

public class PreBuildManifestUpdate : IPostGenerateGradleAndroidProject {

    /// <summary>
    /// Method runs after gradle generated, before build
    /// </summary>
    public void OnPostGenerateGradleAndroidProject(string basePath) {
        AndroidManifest manifest = new AndroidManifest(GetManifestPath(basePath));
        manifest.SetHardwareAcceleration();
        manifest.Save();
    }

    /// <summary>
    /// Callback
    /// </summary>
    public int callbackOrder {
        get {
            return 1;
        }
    }

    /// <summary>
    /// Cached manifest path
    /// </summary>
    private string _manifestFilePath = null;

    /// <summary>
    /// Get manifest file
    /// </summary>
    private string GetManifestPath(string basePath) {
        if (string.IsNullOrEmpty(_manifestFilePath)) {
            StringBuilder pathBuilder = new StringBuilder(basePath);
            pathBuilder.Append(Path.DirectorySeparatorChar).Append("src");
            pathBuilder.Append(Path.DirectorySeparatorChar).Append("main");
            pathBuilder.Append(Path.DirectorySeparatorChar).Append("AndroidManifest.xml");
            _manifestFilePath = pathBuilder.ToString();
        }
        return _manifestFilePath;
    }
}

internal class AndroidXmlDocument : XmlDocument {

    /// <summary>
    /// Cached path
    /// </summary>
    private string _path = null;

    /// <summary>
    /// Namespace manager
    /// </summary>
    protected XmlNamespaceManager _namespaceManager = null;

    /// <summary>
    /// XML namespace
    /// </summary>
    public readonly string AndroidXmlNamespace = "http://schemas.android.com/apk/res/android";

    /// <summary>
    /// Constructor
    /// </summary>
    /// <param name="path">Path for document</param>
    public AndroidXmlDocument(string path) {
        _path = path;
        using (var reader = new XmlTextReader(_path)) {
            reader.Read();
            Load(reader);
        }
        _namespaceManager = new XmlNamespaceManager(NameTable);
        _namespaceManager.AddNamespace("android", AndroidXmlNamespace);
    }

    /// <summary>
    /// Save file
    /// </summary>
    public string Save() {
        return SaveAs(_path);
    }

    /// <summary>
    /// Save file as
    /// </summary>
    /// <param name="path">Path to save</param>
    public string SaveAs(string path) {
        using (XmlTextWriter writer = new XmlTextWriter(path, new UTF8Encoding(false))) {
            writer.Formatting = Formatting.Indented;
            Save(writer);
        }
        return path;
    }
}

internal class AndroidManifest : AndroidXmlDocument {

    /// <summary>
    /// Application element
    /// </summary>
    private readonly XmlElement _application = null;

    /// <summary>
    /// Constructor
    /// </summary>
    /// <param name="path">Path to manifest</param>
    public AndroidManifest(string path) : base(path) {
        _application = SelectSingleNode("/manifest/application") as XmlElement;
    }

    /// <summary>
    /// Create new attribute to manifest
    /// </summary>
    private XmlAttribute CreateAndroidAttribute(string key, string value) {
        XmlAttribute attr = CreateAttribute("android", key, AndroidXmlNamespace);
        attr.Value = value;
        return attr;
    }

    /// <summary>
    /// Get main activity
    /// </summary>
    internal XmlNode GetActivityWithLaunchIntent() {
        return SelectSingleNode("/manifest/application/activity[intent-filter/action/@android:name='android.intent.action.MAIN' and intent-filter/category/@android:name='android.intent.category.LAUNCHER']", _namespaceManager);
    }

    /// <summary>
    /// Enable hardware acceleration
    /// </summary>
    internal void SetHardwareAcceleration() {
        Logger.Info("Set hardware acceleration done");
        GetActivityWithLaunchIntent().Attributes.Append(CreateAndroidAttribute("hardwareAccelerated", "true"));
    }

}