leaudio: Simplify stream configuration

With this patch, group->stream_conf is modified only when CIS is created
or removed. There is no need to recreated it when getting configuration.

This is a preparation for offloader handling
Bug: 231084798
Sponsor: @siyuanh
Test: atest BluetoothInstrumentationTests
Tag: #feature

Change-Id: Ibfaf95387f15893c7bb6c5c19669695c0a478b5e