Fix issue with icon being loaded by the wrong context

- Fixing a regression from ag/12184846 (Reduce ContextThemeWrapper
  usage), the context used to load the icons (which references a
  color in the theme) was not the light context.  This was only an
  issue with ContextualButtons which loaded their icons separate
  from nav bar view

Bug: 162784729
Test: Verify ime switcher and accessibility buttons are visible
Test: atest SystemUITests:NavigationBarContextTest
Change-Id: I7e18cd68889b2b4215fc817becd6412a466c8da5