[Material] [Android, iOS] Materializing the stepper (#5027)

[Material] [Android, iOS] Materializing the stepper (#5027)

* [Android] [Material] Refactored the Stepper on Android #5011
 - reuse code between default and material
 - added material stepper

* [Android] [Material] Using the "Fast" renderer style for the stepper #5011
 - switched the button style to the outlined buttons

* [Android] [Material] Adding the tab stops support (#5000)

* [Android] [Material] Switching back to a traditional renderer for now to get tabstops working

* [Material] [iOS] Added a Material stepper for iOS

* [Material] [Android, iOS] Addressed the PR feedback

* [Material] Remove private

* Update MaterialStepperRenderer.cs

* Update StepperRendererManager.cs

* Fixed the merge issues

* add steppers to visual gallery

* The material stepper buttons now resize to fit the available space.

* Update Xamarin.Forms.Controls/ControlGalleryPages/VisualGallery.xaml

Co-Authored-By: mattleibow <mattleibow@live.com>
* [Material] [Stepper] Don't set the background color for steppers
 - we still need to decide on the logic behind the coloring of outline buttons

* Fixed the Android accessibility

* Change UI tests to not use a Text search for locating stepper controls