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