audio: Preserve 'usecase' for in IModule::setAudioPortConfig

The field AudioPortConfig.ext.mix.usecase is provided by
the client, thus is needs to be treated the same way as
'.handle'.

Bug: 302736184
Test: atest VtsHalAudioCoreTargetTest
Change-Id: I45050e8530cd236f5e582585f583df477fc7ae8a