Implement light mode for persistent Taskbar.

NavbarButtonsViewController has been massaged to make sure the buttons
contrast well with the color of the Taskbar background that they are on,
if applicable.

Test: Manual
Fix: 268052229
Change-Id: I917a1a1be013c304910b0f674ae8a13abb8e47a1
Merged-In: I917a1a1be013c304910b0f674ae8a13abb8e47a1