adds dismiss to CustomActionSheet

[mlisikbf]

adds a dismiss method on CustomActionSheet so that the sheet can be programmatically dismissed, for instance:

const {showActionSheetWithOptions, dismiss} = useActionSheet()

// dismiss action sheet when screen unmounts
useEffect(() => {
  return () => {
     dismiss()
  }
}, [dismiss])

fixes #320
this also previously came up in #104

one use case is dismissing the action sheet on navigation, for instance:

• user opens the sheet
• they background the app
• they come back
• their session timed out
• they're kicked out to the sign in screen
• action sheet persists on top

or:

• user opens the sheet
• they background the app
• they use a deep link
• app opens and navigates
• action sheet persists on top

NOTE: this is a noop on iOS at the moment as there's an old react-native / types dependency and updating that to a version that includes ActionSheetIOS.dismissActionSheet requires additional changes (breaks the build basically)