fix theme preference defaults (#4061)

closes #4060 

Also I noticed that we had the theme defaults twice in the code so I
refactored a bit so only one are still in there.

app/src/main/java/com/keylesspalace/tusky/settings/SettingsConstants.kt

@@ -10,6 +10,9 @@ enum class AppTheme(val value: String) {
 
     companion object {
         fun stringValues() = values().map { it.value }.toTypedArray()
+
+        @JvmField
+        val DEFAULT = AUTO_SYSTEM
     }
 }