Ionic Firebase push notification icon is not visible on all devices

Question

I have an Ionic app (v 2.0.2) consisting Firebase Push notification. I am having some issues. Stating them here.

I am getting the Notification Icon to appear on Xiaomi device but can not get it to show on Samsung. I have used following php code

$message_for_push = array(
    "data" => array(
        "title" => "New Content Published!",
        "body"  => $message['title'],
        "icon"  => "file://assets/img/firebase_push_icon.png", //also tried "icon"  => "myicon",
        "color"  => "#e81010",
        'sound' => 'mySound',

        "type" => $message['type'],
        "section_id" => $message['section_id'],
        "isPush" => 1,
        "subdomain" => $subdomain, //dynamiic
        "id" => $message['id'],
        "other_key" => true
    ),
    "registration_ids" =>  $registrationIds
);

it takes the default icon (not transparent) on Xiaomi but not on Samsung.

I have generated the icons from http://romannurik.github.io/AndroidAssetStudio/icons-notification.html#source.type=clipart&source.clipart=ac_unit&source.space.trim=1&source.space.pad=0&name=ic_stat_ac_unit and put all in mipmap-* folders having name firebase_push_icon.png.

I have also tried with the transparent Icon, does not work either. (I found that I have to provide transparent icon)

Only shows a round circle.

My questions are

1. where should I put the transparent Icon?
2. what should be the file name of the Icon?
3. what should I put on "icon" in php end?
4. When I use both data and notification, I get two notification, why is this?

Thanks in advance.