image_picker: Use FileProvider subclass to avoid collisions

[sir-boformer]

Fixes flutter/flutter#20481

There can only be one declaration for each FileProvider subclass. That means to declare multiple file providers without collisions, one must create a (empty) subclass of android.support.v4.content.FileProvider and use that as the provider name in the manifest.

In my opinion, android.support.v4.content.FileProvider should be reserved for the app itself so that developers can easily add a file provider to their app, without subclassing FileProvider. It's also used in the Android documentation and in StackOverflow answers.

image_picker and other plugins should subclass FileProvider to prevent collisions (also collisions between plugins).

This issue describes a similar problem in an Android library.

[goderbauer]

LGTM