Remove AudioPortConfigType from AIDL interfaces

Switch to the canonical approach of making a field '@nullable'
if the value for it is not required. This makes AudioPortConfigType
redundant.

Refactor affected parceling code in libaudiofoundation to reuse
functions from AidlConversion.

Bug: 198812639
Test: atest audiofoundation_parcelable_test
Change-Id: If02a9d788e7aed21bca917ac193cf71598804b83