To enable bluetooth on Settings UI. you should do this:
(1) commit "<feature name="android.settings.bluetooth" />" in /system/etc/permissions/settings_disabled_menu_list.xml
(2) create file /system/etc/permissions/android.hardware.bluetooth.xml with
<?xml version="1.0" encoding="utf-8"?>
<permissions>
<feature name="android.hardware.bluetooth" />
</permissions>