I found that in ArcGIS runtime for Android 100. x, the style of Callout can be set through Callout.Style,
just like this:
mapView.style = Callout.Style(context, R.xml.callout)
, but in ArcGIS runtime for Android 200. x, there is no way to modify the style externally. Is this part of the functionality not yet released or does it need to be set up through other ways?