Update icon tint color when the user toggles Dark theme

- Reload theme in slice provider when Dark theme mode changes for slices
- Reload theme in onCreate of Panel activity for its non-slice header
- Remove applyTheme from individual slices

Test: robotest
Fixes: 153700819
Change-Id: I40a7d2817c4b9100d7b2f2962a69c8a9ce6f7906