Refactor AudioIoDescriptor

The following changes were made:

 1. Make the class mostly immutable (except for the 'mPatch' field).
 2. Provide constructors for specific use cases.
 3. Add 'isInput' field instead of deducing it by patch data.
 4. Provide conversion to string for logging.
 5. Rename 'audio_io_config_event' to 'audio_io_config_event_t'
    to conform to other enum type names.

Bug: 188932434
Test: m
Change-Id: I35e5c83bcb917f60d01ee977eef9b869c3894074