how to get post meta where value is an array of key value pairs

The meta_query cannot be used to search value which are stored as serialized arrays. You can use it to search multiple values (meta-fields can have several values using the same key).

You need a different approach to solve your problem, this is what I suggest,

store your meta field with the user id as part of the key name,

add_post_meta($review_post_id, 'key_of_post_meta_'.$user_id, $arr );

next, retrieve your posts with,

$args = array(
     'post_type' => 'as_reviews',
     'posts_per_page' => -1,
     'post_status' => 'publish',
     'meta_key' => 'key_of_post_meta_'.$user_id
    );
  $query = new WP_Query($args);