feat: add label on navigate up button

Through tests on system settings app and system messages app, a user hears Navigate up when he clicks on back button. So I used this as a label on back button.

Author: Gautam-Arora24

src/nav/ChatNavBar.js

@@ -62,6 +62,7 @@ class ChatNavBar extends PureComponent<Props> {
         >
           <NavButton
             name="arrow-left"
+            accessibilityLabel="Navigate up"
             color={color}
             onPress={() => {
               NavigationService.dispatch(navigateBack());

src/nav/ModalNavBar.js

@@ -44,6 +44,7 @@ class ModalNavBar extends PureComponent<Props> {
         {canGoBack && (
           <NavButton
             name="arrow-left"
+            accessibilityLabel="Navigate up"
             onPress={() => {
               NavigationService.dispatch(navigateBack());
             }}

src/nav/ModalSearchNavBar.js

@@ -39,6 +39,7 @@ class ModalSearchNavBar extends PureComponent<Props> {
         {canGoBack && (
           <NavButton
             name="arrow-left"
+            accessibilityLabel="Navigate up"
             onPress={() => {
               NavigationService.dispatch(navigateBack());
             }}