Summary:
On Android it's generally expected that alerts are dismissible by tapping outside the alert box. This is the default behavior for React Native, but until now there has been no means to listen for the dismiss event.
`Alert.alert` already takes an `options` object, so this pull request simply adds an additional option `onDismiss`, a callback function that will be called when an Alert is dismissed (rather than being closed as a result of a button press).
**Test plan**
Simply pass an `options` object to `Alert.alert` with the `onDismiss` property set to a function.
e.g. Run the following on Android, dismiss the alert by tapping outside the alert view, and monitor console output.
```
Alert.alert(
'Alert Title',
'My Alert Msg',
[
{text: 'Ask me later', onPress: () => console.log('Ask me later pressed')},
{text: 'Cancel', onPress: () => console.log('Cancel Pressed'), style: 'cancel'},
{text: 'OK', onPress: () => console.log('OK Pressed')},
],
{ onDismiss: () => console.
Closes https://github.com/facebook/react-native/pull/12594
Differential Revision: D4619862
Pulled By: ericvicenti
fbshipit-source-id: fdd351a7b8e673dab331f0e22dc5ea2e84f24375
27928f3993