Summary:
This PR follows the work started in #6431 but instead of implementing the snapshot for Android, will just allow that feature to be implemented by a third party library.
The missing feature is the ability to resolve a View for a given tag integer. which is now possible with a `addUIBlock` on the `UIManagerModule` method where you give a `UIBlock` instance (a new class) that implements a `execute(NativeViewHierarchyManager)` function.
This is already possible in iOS API. a third party can use the `addUIBlock` too.
I have kept the name `addUIBlock` so it's the same as in the [iOS' API](https://github.com/facebook/react-native/search?q=addUIBlock&type=Code&utf8=%E2%9C%93).
---
With this PR a third party lib can now do...
```java
UIManagerModule uiManager = reactContext.getNativeModule(UIManagerModule.class);
uiManager.addUIBlock(new UIBlock() {
public void execute (NativeViewHierarchyManager nvhm) {
View view = nvhm.resolveView(tag);
...do something with view... like... screenshot t
Closes https://github.com/facebook/react-native/pull/8217
Differential Revision: D3469311
Pulled By: astreet
fbshipit-source-id: bb56ecc7e8936299337af47ca8114875ee1fd2b0
Gaëtan Renaudeau