How do I get the current edit page ID in the admin?

Most solutions I found are for front-end use. This is for a plugin, so the all activity is in the back-end.

How do I get the page ID that’s being currently use(edited) in the admin?

Note I’m out of the loop. I just need to get the ID of page(not posts) that I’m currently seeing in the back-end.

You can also use

$post_id = $_GET['post'];

Or you can use a hook (probably better).

function id_WPSE_114111() {
    global $post;
    $id = $post->ID;
    // do something
}

add_action( 'admin_notices', 'id_WPSE_114111' );

You will need to add a conditional since this will run on all admin pages, I recommend using get_current_screen();

For example to run only on pages:

function id_WPSE_114111() {

    global $my_admin_page;
    $screen = get_current_screen();

    if ( is_admin() && ($screen->id == 'page') ) {
        global $post;
        $id = $post->ID;
        var_dump($id);
    }
}

add_action( 'admin_notices', 'id_WPSE_114111' );