[RN] Make header button same size as header label

react/features/base/react/components/native/headerstyles.js

 import { ColorSchemeRegistry, schemeColor } from '../../../color-scheme';
 import { BoxModel } from '../../../styles';
 
+const HEADER_FONT_SIZE = 18;
 const HEADER_HEIGHT = 48;
 
 export const HEADER_PADDING = BoxModel.padding / 2;
 
     headerButtonText: {
         color: schemeColor('text'),
-        fontSize: 20
+        fontSize: HEADER_FONT_SIZE
     },
 
     /**
      */
     headerText: {
         color: schemeColor('text'),
-        fontSize: 18
+        fontSize: HEADER_FONT_SIZE
     },
 
     headerTextWrapper: {