Use native ref instead of tag to focus and blur

This fixes the console error:
focusTextInput must be called with a host component. Passing a react tag is deprecated.

packages/react-native-aztec/src/AztecView.js

@@ -17,6 +17,8 @@ import { ENTER, BACKSPACE } from '@wordpress/keycodes';
 const AztecManager = UIManager.getViewManagerConfig( 'RCTAztecView' );
 
 class AztecView extends React.Component {
+	nativeComponentRef = React.createRef();
+
 	constructor() {
 		super( ...arguments );
 		this._onContentSizeChange = this._onContentSizeChange.bind( this );
@@ -125,7 +127,7 @@ class AztecView extends React.Component {
 
 	_onBlur( event ) {
 		this.selectionEndCaretY = null;
-		TextInputState.blurTextInput( ReactNative.findNodeHandle( this ) );
+		TextInputState.blurTextInput( this.nativeComponentRef.current );
 
 		if ( ! this.props.onBlur ) {
 			return;
@@ -177,11 +179,11 @@ class AztecView extends React.Component {
 	}
 
 	blur() {
-		TextInputState.blurTextInput( ReactNative.findNodeHandle( this ) );
+		TextInputState.blurTextInput( this.nativeComponentRef.current );
 	}
 
 	focus() {
-		TextInputState.focusTextInput( ReactNative.findNodeHandle( this ) );
+		TextInputState.focusTextInput( this.nativeComponentRef.current );
 	}
 
 	isFocused() {
@@ -244,6 +246,7 @@ class AztecView extends React.Component {
 					// combination generate an infinite loop as described in https://github.com/wordpress-mobile/gutenberg-mobile/issues/302
 					onFocus={ this._onAztecFocus }
 					onBlur={ this._onBlur }
+					ref={ this.nativeComponentRef }
 				/>
 			</TouchableWithoutFeedback>
 		);