Android: Implement cancelable option for Alerts

Summary:
**Motivation**
In iOS you cannot dismiss alerts by clicking outside of their box, while on Android you can. This can create some inconsistency if you want to have identical behavior on both platforms. This change makes it possible for Android apps to have irremovable/required alert boxes just like in iOS.

This adds an additional parameter to the Alert method. The way to use it is by providing an object with the cancelable property. The cancelable property accepts a boolean value.

This utilizes the Android DialogFragment method [setCancelable](https://developer.android.com/reference/android/app/DialogFragment.html#setCancelable(boolean))

**Usage example**
```js
Alert.alert(
   'Alert Title',
   null,
   [
     {text: 'OK', onPress: () => console.log('OK Pressed!')},
   ],
   {
     cancelable: false
   }
);
```

**Test plan (required)**

I added an additional alert to the UIExplorer project where it can be tested. I also added a part in the Dialog Module test to make sure setting canc
Closes https://github.com/facebook/react-native/pull/8652

Differential Revision: D3690093

fbshipit-source-id: 4cf6cfc56f464b37ce88451acf33413393454721

ReactAndroid/src/test/java/com/facebook/react/modules/dialog/DialogModuleTest.java

@@ -90,11 +90,13 @@ public class DialogModuleTest {
     options.putString("buttonPositive", "OK");
     options.putString("buttonNegative", "Cancel");
     options.putString("buttonNeutral", "Later");
+    options.putBoolean("cancelable", false);
 
     mDialogModule.showAlert(options, null, null);
 
     final AlertFragment fragment = getFragment();
     assertNotNull("Fragment was not displayed", fragment);
+    assertEquals(false, fragment.isCancelable());
 
     final AlertDialog dialog = (AlertDialog) fragment.getDialog();
     assertEquals("OK", dialog.getButton(DialogInterface.BUTTON_POSITIVE).getText().toString());