fix v5 imports for file-transfer plugin

[nicolashenry]

Imports in v5 are broken for file-transfer plugin, for example :

import { IonicNativePlugin, instanceAvailability, cordovaInstance } from '@ionic-native/core';

which is wrong because the checkAvailability function import is removed from the original file which break FileTransferObject constructor.

It should be :

import { IonicNativePlugin, instanceAvailability, cordovaInstance, checkAvailability } from '@ionic-native/core';