[codec] use_aac_decoder_test asan fix

For now, work around a bug in SoftAAC2.cpp where it can editItemAt(0)
and dereference the returned reference despite the vector being empty.
For now, we let SoftAAC2.cpp have a reference to a dummy_.

MTWN-236 #done

Tested: use_aac_decoder_test (asan)
Change-Id: Ic8effb3ca703530133f98f40bb79bc973594166c