Create new DesktopNavbarButtonsViewController class.

This CL creates a new DesktopNavbarButtonsViewController class to allow
for taskbar buttons and status icons customization on desktop
environment.

Recall: http://recall/clips/e008d2de-36d3-47f6-954a-5ce18bac69a5

Bug: 213964564
Bug: 196757951
Test: Switch between different system navigation modes and verify the
desktop taskbar buttons stay on screen.

Change-Id: Ie7a9a044fad3b60cd37d910b8eb78c8734aadfd5

quickstep/src/com/android/launcher3/taskbar/TaskbarNavButtonController.java

@@ -143,6 +143,10 @@ public class TaskbarNavButtonController {
                 return R.string.taskbar_button_ime_switcher;
             case BUTTON_RECENTS:
                 return R.string.taskbar_button_recents;
+            case BUTTON_NOTIFICATIONS:
+                return R.string.taskbar_button_notifications;
+            case BUTTON_QUICK_SETTINGS:
+                return R.string.taskbar_button_quick_settings;
             default:
                 return 0;
         }