Floss: prepare AAC implementation

In this patch, we prepare the AAC implementation using FFmpeg.

This is not built nor linked by default.

Follow-up work is expected to sandbox the library.

Bug: 268424393
Tag: #floss
Test: atest net_test_stack_a2dp_native

Change-Id: I30987d2decffc3e0d2863d2323d9f62cb4b93f68