NavButton: Use standard icon size of 24, instead of using 26.

Like we did in 2e5ff37. As mentioned there:

"""
As Greg points out [1], 24px seems to be the standard size for most
icon buttons in Material Design [2] [3] [4].

[1] zulip#4442 (comment)
[2] https://material.io/components/bottom-navigation#specs
[3] https://material.io/components/app-bars-bottom#specs
[4] https://material.io/design/iconography/system-icons.html#grid-and-keyline-shapes
"""

Author: chrisbobbe

src/nav/NavButton.js

@@ -31,7 +31,7 @@ export default class NavButton extends PureComponent<Props> {
 
     return (
       <NavButtonGeneral onPress={onPress} accessibilityLabel={accessibilityLabel}>
-        <Icon size={26} style={[this.styles.navButtonIcon, style]} color={color} name={name} />
+        <Icon size={24} style={[this.styles.navButtonIcon, style]} color={color} name={name} />
       </NavButtonGeneral>
     );
   }