Use Locale default temperature unit in wallpaper picker

The assistant temperature unit is not available when rendering the
clock in the wallpaper picker context. We mitigate this by using
the locale default instead to determine which temperature unit should
be shown.

Bug: 285621525
Test: Manually checked several different locales
Change-Id: I826cfb7dd424d776dad5f159341595711cf90727
Merged-In: I826cfb7dd424d776dad5f159341595711cf90727