Fix NRE when using Custom Slide Transition

[PureWeen]

Description of Change

ShellFlyoutRenderer wasn't correctly checking for scenarios where users are implementing a Custom Slide Flyout Transition class which was causing an NRE. I've also set the LayoutViews method to be virtual and add made UpdateFlyoutSize public so users can reuse SlideFlyoutTransition

Issues Resolved

• fixes [Bug] SlideFlyoutTransition is not working after the xamarin forms update to xamarin forms 5  #13390

API Changes

Added:

public class SlideFlyoutTransition 
     public virtual bool UpdateFlyoutSize(double height, double width)

Changed:

public class SlideFlyoutTransition 
     public virtual void LayoutViews(CGRect bounds, nfloat openPercent, UIView flyout, UIView shell, FlyoutBehavior behavior)

Platforms Affected

• iOS

Testing Procedure

• ui test included

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)