Fix drag-and-drop start gesture

[m-sasha]

Our current DragAndDropSourceDefaults.DefaultStartDetector does detectTapGestures(onLongPress=...), which does two things wrong:

1. It triggers on long-press with a mouse.
2. It does not trigger on drag-start with a mouse.

This PR fixes these two issues.

Testing

• Tested manually (only with a mouse) on

@OptIn(ExperimentalComposeUiApi::class)
fun main() = singleWindowApplication {
    val exportedText = "Hello, DnD!"
    Row(
        horizontalArrangement = Arrangement.SpaceEvenly,
        verticalAlignment = Alignment.CenterVertically,
        modifier = Modifier.fillMaxSize()
    ) {
        val textMeasurer = rememberTextMeasurer()
        Box(Modifier
            .size(200.dp)
            .background(Color.LightGray)
            .dragAndDropSource(
                drawDragDecoration = {
                    drawRect(
                        color = Color.White,
                        topLeft = Offset(x = 0f, y = size.height/4),
                        size = Size(size.width, size.height/2)
                    )
                    val textLayoutResult = textMeasurer.measure(
                        text = AnnotatedString(exportedText),
                        layoutDirection = layoutDirection,
                        density = this
                    )
                    drawText(
                        textLayoutResult = textLayoutResult,
                        topLeft = Offset(
                            x = (size.width - textLayoutResult.size.width) / 2,
                            y = (size.height - textLayoutResult.size.height) / 2,
                        )
                    )
                },
                transferData = { offset ->
                    DragAndDropTransferData(
                        transferable = DragAndDropTransferable(
                            StringSelection(exportedText)
                        ),
                        supportedActions = listOf(
                            DragAndDropTransferAction.Copy,
                            DragAndDropTransferAction.Move,
                            DragAndDropTransferAction.Link,
                        ),
                        dragDecorationOffset = offset,
                        onTransferCompleted = { action ->
                            println("Action at source: $action")
                        }
                    )
                }
            )
        ) {
            Text("Drag Me", Modifier.align(Alignment.Center))
        }
    }
}

This should be tested by QA to make sure that it works for touch.

[MatkovIvan]

The change LGTM. Can we add auto test for this?

[m-sasha]

The change LGTM. Can we add auto test for this?

I'll do it in a separate PR, since it requires to first add support for drag-and-drop in tests.

[m-sasha]

BTW, actually you can't do

pointerInput(Unit) {
   oneDetector()
   anotherDetector()
}

because the detectors never return. Fixed by wrapping them in a coroutine scope with launch.