fix incorrectly selected current theme in the theme radio (#6182)

Task/Issue URL:
https://app.asana.com/1/137249556945/project/414730916066338/task/1210402384079670?focus=true

### Description
Fixes the theme radio to pre-select correct theme.

### Steps to test this PR

- [ ] Open Settings -> Appearance -> Theme.
  - [ ] Verify that your current theme is pre-selected correctly.
  - [ ] Change theme and try again.
- [ ] Toggle on the Experimental UI Settings -> Enable Visual design
updates.
- [ ] Verify that your current theme (light/dark) is pre-selected
correctly. Previously, it would pre-select nothing.
  - [ ] Change theme and try again.

Author: LukasPaczos

common/common-ui/src/main/java/com/duckduckgo/common/ui/Theming.kt

@@ -42,10 +42,8 @@ enum class DuckDuckGoTheme {
     fun getOptionIndex(): Int {
         return when (this) {
             SYSTEM_DEFAULT -> 1
-            DARK -> 2
-            LIGHT -> 3
-            EXPERIMENT_DARK -> 4
-            EXPERIMENT_LIGHT -> 5
+            LIGHT, EXPERIMENT_LIGHT -> 2
+            DARK, EXPERIMENT_DARK -> 3
         }
     }
 }