audio_utils: Fix 7.1.4 channel mix TOP_BACK_LEFT/RIGHT

Should go into the same destination channel, not
BACK_LEFT/BACK_RIGHT.

Add a test to catch the issue.

Test: atest channels_tests
Bug: 323277496
Merged-In: Ib512179c341d9cff264318428ad140364504da9f
Change-Id: Ib512179c341d9cff264318428ad140364504da9f
2 files changed