Raised Middle Button TabbedPage implementation for Xamarin.Android. Just replace the `Replace_With_Your_Custom_BottomNavigationView` with your own implementation of BNV!

RaisedTabbedPageRenderer.cs

using System;
using System.Reflection;
using Android.Content;
using Android.Support.Design.Widget;
using Android.Support.V4.View;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;
using AWidget = Android.Widget;
using ATabbedRenderer = Xamarin.Forms.Platform.Android.AppCompat.TabbedPageRenderer;
using System.Collections.Specialized;

namespace Your.Namespace.Here
{
    public class RaisedTabbedPageRenderer : ATabbedRenderer
    {
        AWidget.RelativeLayout InternalRelativeLayout
        {
            get
            {
                var field = typeof(ATabbedRenderer).GetField("_relativeLayout", BindingFlags.NonPublic | BindingFlags.Instance);
                return (AWidget.RelativeLayout)field.GetValue(this);
            }
            set
            {
                var field = typeof(ATabbedRenderer).GetField("_relativeLayout", BindingFlags.NonPublic | BindingFlags.Instance);
                field.SetValue(this, value);
            }
        }

        BottomNavigationView InternalBottomNavigationView
        {
            get
            {
                var field = typeof(ATabbedRenderer).GetField("_bottomNavigationView", BindingFlags.NonPublic | BindingFlags.Instance);
                return (BottomNavigationView)field.GetValue(this);
            }
            set
            {
                var field = typeof(ATabbedRenderer).GetField("_bottomNavigationView", BindingFlags.NonPublic | BindingFlags.Instance);
                field.SetValue(this, value);
            }
        }

        ViewPager InternalViewPager
        {
            get
            {
                var field = typeof(ATabbedRenderer).GetField("_viewPager", BindingFlags.NonPublic | BindingFlags.GetField | BindingFlags.Instance);
                return (ViewPager)field.GetValue(this);
            }
            set
            {
                var field = typeof(ATabbedRenderer).GetField("_viewPager", BindingFlags.NonPublic | BindingFlags.Instance);
                field.SetValue(this, value);
            }
        }

        public RaisedTabbedPageRenderer(Context context) : base(context)
        {

        }

        protected override void OnElementChanged(ElementChangedEventArgs<TabbedPage> e)
        {
            base.OnElementChanged(e);

            PerformHack(e);
        }

        void PerformHack(ElementChangedEventArgs<TabbedPage> e)
        {
            RemoveView(InternalRelativeLayout);

            InternalRelativeLayout = new AWidget.RelativeLayout(Context)
            {
                LayoutParameters = new LayoutParams(LayoutParams.MatchParent, LayoutParams.MatchParent)
            };

            if (InternalBottomNavigationView != null)
            {
                InternalRelativeLayout.RemoveView(InternalBottomNavigationView);
                InternalBottomNavigationView.SetOnNavigationItemSelectedListener(null);
            }

            var bottomNavigationViewLayoutParams = new AWidget.RelativeLayout.LayoutParams(
                LayoutParams.MatchParent,
                LayoutParams.WrapContent);

            bottomNavigationViewLayoutParams.AddRule(AWidget.LayoutRules.AlignParentBottom);

            // TODO Replace with your custom BottomNavigationView
            InternalBottomNavigationView = new RaisedBottomNavigationBar(Context)
            {
                LayoutParameters = bottomNavigationViewLayoutParams,
                Id = GenerateViewId()
            };

            var viewPagerParams = new AWidget.RelativeLayout.LayoutParams(LayoutParams.MatchParent, LayoutParams.MatchParent);
            viewPagerParams.AddRule(AWidget.LayoutRules.Above, InternalBottomNavigationView.Id);

            var pager = InternalViewPager;

            pager.RemoveFromParent();

            InternalRelativeLayout.AddView(pager, viewPagerParams);
            InternalRelativeLayout.AddView(InternalBottomNavigationView, bottomNavigationViewLayoutParams);

            AddView(InternalRelativeLayout);

            OnChildrenCollectionChangedInternal(null, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));

            TabbedPage tabbedPage = e.NewElement;
            if (tabbedPage.CurrentPage != null)
                InvokeMethod("ScrollToCurrentPage");

            InvokeMethod("UpdateBarBackgroundColor");
            InvokeMethod("UpdateBarTextColor");
            InvokeMethod("UpdateItemIconColor");
            InvokeMethod("UpdateSwipePaging");
            InvokeMethod("UpdateOffscreenPageLimit");
        }

        void OnChildrenCollectionChangedInternal(object sender, NotifyCollectionChangedEventArgs e)
        {
            MethodInfo privMethod = typeof(ATabbedRenderer).GetMethod("OnChildrenCollectionChanged", BindingFlags.NonPublic | BindingFlags.Instance);
            privMethod.Invoke(this, new object[] { sender, e });
        }

        void InvokeMethod(string methodName)
        {
            MethodInfo privMethod = typeof(ATabbedRenderer).GetMethod(methodName, BindingFlags.NonPublic | BindingFlags.Instance);
            privMethod.Invoke(this, new object[] { });
        }
    }
}