Effect VTS: update VtsHalDownmixTargetTest for data validation

For downmix, the output buffer size can be diff with input

Bug: 318926783
Bug: 317946442
Test: atest --test-mapping hardware/interfaces/audio/aidl/vts:presubmit
Change-Id: I0dc0009e1779b842a4f3cdcc047d225310a304f9
Merged-In: I0dc0009e1779b842a4f3cdcc047d225310a304f9
1 file changed