When Unity3D stubbornly resets / forgets your Android app package name...

SetAppBundleIdentifierContinuously.cs

// Put this script in your "Editor" scripts folder ;)

#if UNITY_EDITOR

using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
class SetAppBundleIdentifierContinuously
{
  static readonly string bundleIdentifier   = "com.my.package.name";

  static readonly BuildTargetGroup[] buildTargetGroups = 
  {
    BuildTargetGroup.Standalone, 
    BuildTargetGroup.Android, 
    BuildTargetGroup.iOS
  };

  static SetAppBundleIdentifierContinuously ()
  {
      EditorApplication.update += Update;
  }

  static void Update ()
  {
    SetApplicationIdentifierForBuildTargetGroups(bundleIdentifier);
    
    if (hello_count < 0)
    {
      Debug.Log("Hello Unity...");
      hello_count = 200;
    }
    
    hello_count--;
  }

	static void SetApplicationIdentifierForBuildTargetGroups(string application_identifier)
	{
		foreach (var build_target_group in buildTargetGroups)
		{
			//Debug.Log("Setting Application identifier '" + application_identifier + "' for build target group " + build_target_group);
			PlayerSettings.SetApplicationIdentifier(build_target_group, application_identifier);
		}
	}

	static int hello_count;
}

#endif