Bring back TextInput.State, deprecate focusTextInput and blurTextInput

Summary:
a275eac56e removed TextInput.State but we should keep it as it was a public-ish API and we don't have any migration plan off it. Also bring back `focusTextInput` and `blurTextInput` with a deprecation warning.

Tested TextInput.State is back

[GENERAL][ENHANCEMENT][TextInput] - Bring back TextInput.State, deprecate focusTextInput and blurTextInput
Closes https://github.com/facebook/react-native/pull/18936

Differential Revision: D8044439

Pulled By: hramos

fbshipit-source-id: fde145f04bb1d46ef58b5954cb7963adf495b21c

Libraries/Components/TextInput/TextInput.js

@@ -325,7 +325,23 @@ type Props = $ReadOnly<{|
 
 const TextInput = createReactClass({
   displayName: 'TextInput',
-
+  statics: {
+    State: {
+      currentlyFocusedField: TextInputState.currentlyFocusedField,
+      focusTextInput: (textFieldID: ?number) => {
+        console.warn(
+          '`focusTextInput` is deprecated, use the `focus` method of the `TextInput` ref instead.',
+        );
+        TextInputState.focusTextInput(textFieldID);
+      },
+      blurTextInput: (textFieldID: ?number) => {
+        console.warn(
+          '`blurTextInput` is deprecated, use `Keyboard.dismiss` or the `blur` method of the `TextInput` ref.',
+        );
+        TextInputState.blurTextInput(textFieldID);
+      },
+    },
+  },
   propTypes: {
     ...ViewPropTypes,
     /**