Aura App
<aura:application extends="force:slds">
    <c:myContactCmp recordId='0017F00000S7UjN'/>
</aura:application>

Componet 
<aura:component implements="flexipage:availableForRecordHome,force:hasRecordId" controller="myContactController" access="global" >
    <aura:handler name="init" action="{!c.getContactList}" value="{!this}" />
    <aura:attribute name="contactList" type="List"/>
</aura:component>

Component controller 
({
    getContactList: function(component, event, handler) {
        
        var action = component.get("c.ContactsList");
        //var accId = component.get("v.recordId");
        alert('acc id '+ component.get("v.recordId"));
        /*action.setParams({
            accId : accId
        });*/
                action.setParams({ AccountId : component.get("v.recordId") });

        action.setCallback(this, function(response){
            var state = response.getState();
            if(state == 'SUCCESS'){
                var contactList = response.getReturnValue();
                //alert("From server: " + contactList);
               //console.log("From server: " + contactList);
               component.set("v.contactList", contactList);
            }else{
                alert('alert in getting data');
            }
        });
            $A.enqueueAction(action);       
    }
})

Apex Controller

It's shows null recordId 

public class myContactController {
    
    @AuraEnabled 
    public static List<Contact> ContactsList(Id accountId){
        system.debug('acc id is '+accountId);
        List<Contact> contacts = [SELECT ID,name,AccountId,Phone,Email from Contact where accountId =: accountId];
        return contacts;
    }
}