AAC codec (wrapper) unit tests.

Bug: 256013806
Tag: #refactor
Test: atest net_test_stack_a2dp_codecs_native
Ignore-AOSP-First: presubmit fails on AOSP because the OPUS codec only
exists on internal master

Change-Id: Ide7f361aab134a4d2579398ad9673f453454afa0