fix(Android): do not crash in case background can not be casted to Ma…

…terialShapeDrawable (software-mansion#2388)

## Description

It turns out, that this check:
software-mansion@b2dab78
was necessary,
and I forgot about it while applying the code review.

Current code will crash if `background != null && !(background is
MaterialShapeDrawable)`, so e.g. when `background is ColorDrawable`.
We don't wanna fail in this case.

## Changes

Allowed the cast to fail instead of throwing exception, when casting
background to `MaterialShapeDrawable`.
One thing that concerns me 

## Test code and steps to reproduce

`Test1649`

## Checklist

- [ ] Included code example that can be used to test this change
- [ ] Ensured that CI passes

android/src/main/java/com/swmansion/rnscreens/Screen.kt

@@ -441,7 +441,7 @@ class Screen(
         if (stackPresentation !== StackPresentation.FORM_SHEET || background == null) {
             return
         }
-        (background as MaterialShapeDrawable?)?.let {
+        (background as? MaterialShapeDrawable?)?.let {
             val resolvedCornerRadius = PixelUtil.toDIPFromPixel(sheetCornerRadius)
             it.shapeAppearanceModel =
                 ShapeAppearanceModel