Add mock API and reorg code for clarity

src/view/shell/index.tsx

@@ -0,0 +1,12 @@
+import React from 'react'
+import {SafeAreaView} from 'react-native'
+import {isDesktopWeb} from '../../platform/detection'
+import {DesktopWebShell} from './desktop-web/shell'
+
+export const Shell: React.FC = ({children}) => {
+  return isDesktopWeb ? (
+    <DesktopWebShell>{children}</DesktopWebShell>
+  ) : (
+    <SafeAreaView>{children}</SafeAreaView>
+  )
+}