fix(rn) stop room name generator when field is focused

Fixes: https://github.com/jitsi/jitsi-meet/issues/8307

react/features/welcome/components/WelcomePage.native.js

      * @returns {void}
      */
     _onFieldFocusChange(focused) {
-        focused
-            && this.setState({
-                _fieldFocused: true
+        if (focused) {
+            // Stop placeholder animation.
+            this._clearTimeouts();
+            this.setState({
+                _fieldFocused: true,
+                roomPlaceholder: ''
             });
+        } else {
+            // Restart room placeholder animation.
+            this._updateRoomname();
+        }
 
         Animated.timing(
             this.state.hintBoxAnimation,