Refactor settings/preferencesearch/PreferenceSearchItem#allRelevantSearchFields

It doesn't need to return mutable list

Author: Yevhen Babiichuk (DustDFG)

app/src/main/java/org/schabi/newpipe/settings/preferencesearch/PreferenceSearchItem.kt

@@ -30,9 +30,8 @@ data class PreferenceSearchItem(
         return !key.isEmpty() && !title.isEmpty()
     }
 
-    fun getAllRelevantSearchFields(): MutableList<String?> {
-        return mutableListOf(title, summary, entries, breadcrumbs)
-    }
+    val allRelevantSearchFields: List<String>
+        get() = listOf(title, summary, entries, breadcrumbs)
 
     override fun toString(): String {
         return "PreferenceItem: $title $summary $key"