Better UX for separate SDR vs HDR subtitle styles (#1622)

## Description
This PR moves the separated HDR subtitle style behind a toggle. This
means users must now opt-in to using separated SDR vs HDR subtitle
styles. This is a better UX for users who want to customize the style
but use the same styles for both.

Users who have changed the HDR subtitles previously will have the toggle
enabled automatically. If the user hasn't changed HDR style or their SDR
& HDR styles are the the same, the toggle will be disabled.

Also, fixes a bug where when playback switched between SDR or HDR, the
subtitle styles might not be updated.

### Related issues
Closes #1603

### Testing
Emulator & unit tests for migration

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/proto/WholphinDataStore.proto

@@ -141,6 +141,7 @@ message SubtitlePreferences{
   int32 margin = 11;
   int32 edge_thickness = 12;
   int32 image_subtitle_opacity = 13;
+  bool use_separate_hdr = 14;
 }
 
 message LiveTvPreferences {