[Shell,Android] BackButtonBehavior enables FlyoutBehavior

[davidortinau]

Description

When the FlyoutBehavior="Disabled" and then I add a BackButtonBehavior on a ContentPage, the flyout becomes active on Android.

Android: https://www.screencast.com/t/xqjM4PeTrEH

iOS: https://www.screencast.com/t/VpYNt6aRJsj3

Steps to Reproduce

<?xml version="1.0" encoding="UTF-8"?>
<Shell xmlns="http://xamarin.com/schemas/2014/forms" 
       xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
       xmlns:local="clr-namespace:VisualChallenge"
       RouteHost="companyname.com"
       RouteScheme="app"
       Route="VisualChallenge"
       FlyoutBehavior="Disabled"
       Title="VisualChallenge"
       x:Class="VisualChallenge.AppShell"
       Visual="Material"
       >

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:sys="clr-namespace:System;assembly=mscorlib"
             x:Class="VisualChallenge.VisualChallengePage"
             Shell.NavBarIsVisible="true"
             Title="Edit Request"
             >
    
    <ContentPage.ToolbarItems>
        <ToolbarItem Text="Save"/>
    </ContentPage.ToolbarItems>
    
    <Shell.BackButtonBehavior>
        <BackButtonBehavior TextOverride="Undo" IconOverride="{OnPlatform Android=blanky}"></BackButtonBehavior>
    </Shell.BackButtonBehavior>

Expected Behavior

Flyout doesn't become active.

Basic Information

• Version with issue: 3.6-pre1
• Android 26
  Parent: Xamarin.Forms.Shell Spec #2415