Init gain config to prevent uninit leak.

In AudioPortConfig, we only initialize index for audio_gain_config, but
not other fields. That may cause uninit leak at listAudioPorts and
listAudioPatches.

Bug: 77238250
Bug: 77238762
Test: try repo steps at the bug description.
Change-Id: I57e3bd0598f9aa698a6fa3d3c0218b046de34e2f
(cherry picked from commit ebe0777e)