Ensure Android PdfView defaults to transparent background when BackgroundColor is unset

[Copilot]

When PdfView is placed in a Grid on Android, leaving BackgroundColor unset could cause row 0 content to be visually obscured. Explicitly setting BackgroundColor="Transparent" worked around it; this change makes that behavior the default.

• Android native background initialization

  • Set the underlying PDFView background to transparent at construction time so unset MAUI background does not inherit problematic native defaults.

• Consistent null-background mapping

  • Updated BackgroundColor mapping to always apply a native color:
    • explicit MAUI color → mapped ARGB
    • null MAUI background → Android.Graphics.Color.Transparent

• Behavioral impact

  • Preserves explicit background behavior.
  • Removes the need for consumers to manually set BackgroundColor="Transparent" to avoid Grid row 0 visibility issues.

public PdfViewAndroid(Context context)
{
    _pdfView = new PDFView(context, null);
    _pdfView.SetBackgroundColor(global::Android.Graphics.Color.Transparent);
}

public Color? BackgroundColor
{
    get => _backgroundColor;
    set
    {
        _backgroundColor = value;
        var androidColor = value != null
            ? global::Android.Graphics.Color.Argb(
                (int)(value.Alpha * 255),
                (int)(value.Red * 255),
                (int)(value.Green * 255),
                (int)(value.Blue * 255))
            : global::Android.Graphics.Color.Transparent;
        _pdfView.SetBackgroundColor(androidColor);
    }
}