Clean hidden api - FastXmlSerializer

Remove hidden api for mainline project.
This commit is for FastXmlSerializer,
android.util.Xml give a stable XmlSerializer interface.
FYI: Since 2015, the XmlSerializer has a buffer and is twice
as fast as before (aosp/147924). It should compare to the
FastXmlSerializer

Bug: 190443169
Test: atest BluetoothInstrumentationTests:BipImageDescriptorTest
Tag: #refactor
Change-Id: I9495bf8c8f942773f334d1e677f8d0a7ff5fe49f