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/1f3b88e2-9f4e-4291-a9fe-a17641d1fd0c

Bug: 213964564
Bug: 196757951
Test: Switch between different system navigation modes and verify the
desktop taskbar buttons/status icons stay on screen, and are functional.

Change-Id: I05846b61689213ded88b8f49460aced88e49fd65