AsyncImagePainter's placeholder/fallback/error is only updated when a state transition occurs

[colinrtwhite]

Describe the bug
AsyncImagePainter's placeholder/fallback/error values are only updated when a state transition occurs. This is currently semi by design as those values are only used to overwrite the values provided by ImageRequest's placeholder/fallback/error, however in practice this can be unexpected. To figure out:

• If we can update those values outside of state transitions caused by ImageLoader how does this affect onState and when it's called? Do we re-invoke the callback with Loading a second time if placeholder is updated?

To Reproduce
The following composable won't update to blue:

AsyncImage(
    model = Unit, // Pass a value that errors immediately.
    error = changingPainter(),
    contentDescription = null,
)

@Composable
fun changingPainter(): Painter {
    val painter by remember { mutableStateOf<Painter>(ColorPainter(Color.Red)) }
    LaunchedEffect(Unit) {
        delay(1_000)
        painter = ColorPainter(Color.Blue)
    }
    return painter
}

Version
Reproduced on 2.4.0.