Fix bug trying to call a method on a nullptr.

This CL defensively checks for a null mIamfDecoder before dereferencing.

Also, fix bug with new output channel mask requests not being applied.

Bug: 441168307
Flag: EXEMPT Bug fix
Test: atest VtsHalMediaC2V1_0TargetComponentTest
Change-Id: I690ce079bc919325f1d5f427fcfde5e36aff5317