Fix experimental AC3 setting being used by default (#1879)

## Description
There were two problems with the experimental "Use AC3 for surround
sound audio" setting (#947):
1. Defaulted to true
2. Access wasn't guarded by enabling experimental settings

This caused unintentional audio transcoding

### Related issues
Fixes #1878

### Testing
Emulator

## Screenshots
N/A

## AI or LLM usage
None

app/src/main/java/com/github/damontecres/wholphin/preferences/ExperimentalPreference.kt

@@ -40,7 +40,7 @@ object ExperimentalPreference {
     val PreferAc3ForSurround =
         AppSwitchPreference<AppPreferences>(
             title = R.string.prefer_ac3_for_surround,
-            defaultValue = true,
+            defaultValue = false,
             getter = { it.experimentalPreferences.preferAc3Surround },
             setter = { prefs, value ->
                 prefs.updateExperimentalPreferences {