lightbox header/footer: Use size prop.

Greg points out [1] that it doesn't really make sense to set the
size with `fontSize`.

[1] zulip#4442 (comment)

Author: chrisbobbe

src/lightbox/LightboxFooter.js

@@ -21,7 +21,6 @@ const styles = createStyleSheet({
     alignSelf: 'center',
   },
   icon: {
-    fontSize: 28,
     alignSelf: 'center',
   },
 });
@@ -39,7 +38,7 @@ export default class LightboxFooter extends PureComponent<Props> {
       <SafeAreaView mode="padding" edges={['right', 'bottom', 'left']}>
         <View style={[styles.wrapper, style]}>
           <Text style={styles.text}>{displayMessage}</Text>
-          <Icon style={styles.icon} color="white" name="more-vertical" onPress={onOptionsPress} />
+          <Icon size={28} color="white" name="more-vertical" onPress={onOptionsPress} />
         </View>
       </SafeAreaView>
     );

src/lightbox/LightboxHeader.js

@@ -27,9 +27,6 @@ const styles = createStyleSheet({
   rightIconTouchTarget: {
     alignSelf: 'center',
   },
-  rightIcon: {
-    fontSize: 28,
-  },
   wrapper: {
     flex: 1,
     flexDirection: 'row',
@@ -74,7 +71,7 @@ export default class LightboxHeader extends PureComponent<Props> {
             </Text>
           </View>
           <Touchable style={styles.rightIconTouchTarget} onPress={onPressBack}>
-            <Icon style={styles.rightIcon} color="white" name="x" />
+            <Icon size={28} color="white" name="x" />
           </Touchable>
         </View>
       </SafeAreaView>