Fix color option selection indicator

After changing wallpaper, no color option shows as selected. This is
because the optimistic update value that is overriding the actual
selected color option was not reset to null after the update is
complete. In addition to resetting the optimistic update value to null
at the right time, the shareIn function to convert flows to SharedFlows
is removed since it was preventing updates from flowing consistently
from the repository to the view model.

Flag: NA
Test: manually verified that the expected color option is selected after
a wallpaper change
Test: regression tested color picker features including pausing and
resuming app and changing light/dark theme, no regressions observed
Bug: 318050029

Change-Id: Ia26d0d5fd763a55d8421bacd9b9a8ebdb6085862