Fix losing status bar icons on config changes

The DarkIconManager was not correctly calling set when icons
were added, leaving them blank after a config change until a new
set came in.

Do some refactoring to fix this and make it more testable.

Test: runtest systemui
Change-Id: I0b231021f2ce7d82a3f84ebb281b4e4fc902a0aa
Fixes: 35367550