Add support for user-selectable night mode

Adds a Material theme that automatically selects light or dark UI based
on the night mode resource qualifier.

Removes restriction that night mode only works when car mode is enabled
or the device is docked, which was applied inconsistently anyway and
only actually worked when in car mode (regardless of docked state).
Night mode is now always applied according to the system preference.

Change-Id: I526807c907eb29a2ab588d15932afe3e6ab180c2