[fix] Modal z-index

The z-index must be applied to the outer element (or even the portal element)
if the portal is to stack relative to other silbings. Previously, the z-index
was only relative to the outer Modal element which had a non-static position.

Fix #1820

packages/react-native-web/src/exports/Modal/ModalAnimation.js

@@ -79,7 +79,8 @@ const styles = StyleSheet.create({
     top: 0,
     right: 0,
     bottom: 0,
-    left: 0
+    left: 0,
+    zIndex: 9999
   },
   animatedIn: {
     animationDuration: `${ANIMATION_DURATION}ms`,

packages/react-native-web/src/exports/Modal/ModalContent.js

@@ -55,8 +55,7 @@ const styles = StyleSheet.create({
     top: 0,
     right: 0,
     bottom: 0,
-    left: 0,
-    zIndex: 9999
+    left: 0
   },
   modalTransparent: {
     backgroundColor: 'transparent'