serialize method
Serializes the tile widget to translatable tile widget to be handled by android.
Implementation
@override
Map<String, Object> serialize() => {
kTileWidgetType: '__circularprogressindicator',
'progress': progress,
if (startAngle != null) 'startAngle': startAngle!,
if (endAngle != null) 'endAngle': endAngle!,
};