[Android] Update the adapter used by RecyclerViewScrollListener

[rmarinho]

Description of Change

When the ItemsSource/Adapter was changing we weren't updating the reference on the scrollviewer so we could't get the right position.

Did some other cleanup

Issues Fixed

Fixes #6328
Fixes #6457

Use the following page to test - Make sure position changes on load, then change categor on top, position should reset to 0, and swipe again to see if position updates properly.

	public partial class CarouselDemo : ContentPage
	{
		Color GetColor(int page) => page switch
		{
			0 => Colors.DarkRed,
			1 => Colors.DarkGreen,
			2 => Colors.DarkBlue,
			3 => Colors.DarkOrange,
			4 => Colors.DarkOliveGreen,
			_ => null
		};

		public CarouselDemo()
		{
			List<Category> categories = new List<Category>();

			for (int cat = 0; cat < 4; cat++)
			{
				var category = new Category()
				{
					Col = cat,
					Name = $"Category {cat}"
				};

				for (int page = 0; page < 5; page++)
				{
					var items = new List<Item>();

					for (int col = 0; col < 5; col++)
					{
						for (int row = 0; row < 6; row++)
						{
							items.Add(new Item()
							{
								Name = $"Categ {cat}, page {page}",
								Col = col,
								Row = row,
								Color = GetColor(page)
							});
						}
					}

					category.Items.Add(new Pager<Item>(page, items));
				}

				categories.Add(category);
			}

			Categories.Add(new Pager<Category>(0, categories));

			CurrentCategory = Categories[0][0];

			BindingContext = this;
			InitializeComponent();
		}

		public List<Pager<Category>> Categories { get; } = new List<Pager<Category>>();

		private Category currentCategory;
		public Category CurrentCategory
		{
			get => currentCategory;
			set
			{
				currentCategory = value;
				OnPropertyChanged();
			}
		}

		private void CategBtn_Clicked(object sender, EventArgs e)
		{
			if (sender is Button b && b.BindingContext is Category cat)
				CurrentCategory = cat;
		}
	}
	public abstract class BaseClass
	{
		public int Row { get; set; }
		public int Col { get; set; }

		public string Name { get; set; }
	}
	public class Category : BaseClass
	{
		public List<Pager<Item>> Items { get; } = new List<Pager<Item>>();
	}
	public class Item : BaseClass
	{
		public Color Color { get; set; }
	}
	public class Pager<T> : List<T>
	{
		public Pager(int page, IEnumerable<T> items) : base(items)
		{
			PageIndex = page;
		}

		public int PageIndex { get; }
	}

XAML

  <ContentPage.Resources>

        <DataTemplate x:Key="CategView">
            <Grid BindableLayout.ItemsSource="{Binding}" ColumnSpacing="5"
                  ColumnDefinitions="*,*,*,*">

                <BindableLayout.ItemTemplate>

                    <DataTemplate>

                        <Button x:Name="CategBtn"
                                Grid.Column="{Binding Col}"
                                Background="DarkRed"
                                Clicked="CategBtn_Clicked"
                                CornerRadius="0"
                                FontSize="16"
                                LineBreakMode="WordWrap"
                                Text="{Binding Name}" />

                    </DataTemplate>

                </BindableLayout.ItemTemplate>
            </Grid>
        </DataTemplate>

        <DataTemplate x:Key="ItemView">
            <Grid BindableLayout.ItemsSource="{Binding}"
                  ColumnDefinitions="*,*,*,*,*"
                  ColumnSpacing="5"
                  RowDefinitions="*,*,*,*,*,*"
                  RowSpacing="5">

                <BindableLayout.ItemTemplate>

                    <DataTemplate>

                        <Button Grid.Row="{Binding Row}"
                                Grid.Column="{Binding Col}"
                                Background="{Binding Color}"
                                CornerRadius="0"
                                FontSize="14"
                                LineBreakMode="WordWrap"
                                Text="{Binding Name}"
                                TextColor="White" />

                    </DataTemplate>

                </BindableLayout.ItemTemplate>

            </Grid>
        </DataTemplate>

    </ContentPage.Resources>

    <Grid RowDefinitions="Auto,0.25*,*" RowSpacing="5">

        <Label FontSize="24"
               HorizontalTextAlignment="Center"
               TextColor="Green"
               VerticalTextAlignment="Center">
            <Label.Text>
                <MultiBinding StringFormat="Position: {0}">
                    <Binding Path="Position"
                             Source="{x:Reference ItemSwiper}" />
                </MultiBinding>
            </Label.Text>
        </Label>

        <CarouselView Grid.Row="1"
                      ItemTemplate="{StaticResource CategView}"
                      ItemsSource="{Binding Categories}"
                      Loop="False">

            <CarouselView.ItemsLayout>
                <LinearItemsLayout Orientation="Horizontal"
                                   SnapPointsAlignment="Start"
                                   SnapPointsType="MandatorySingle" />
            </CarouselView.ItemsLayout>

        </CarouselView>

        <CarouselView x:Name="ItemSwiper"
                      Grid.Row="2"
                      Background="Yellow"
                      ItemTemplate="{StaticResource ItemView}"
                      ItemsSource="{Binding CurrentCategory.Items}"
                      Loop="False">

            <CarouselView.ItemsLayout>
                <LinearItemsLayout Orientation="Horizontal"
                                   SnapPointsAlignment="Start"
                                   SnapPointsType="MandatorySingle" />
            </CarouselView.ItemsLayout>

        </CarouselView>

    </Grid>
    ``` 

[hartez]

Why are we no longer updating the ItemsSource when it changes?

[rmarinho]

The mapper does this