Audio: SpatializerHelper: fix native audio server death handling

Make sure that the native spatializer interface reference
if cleared when the native audio server dies so that is is properly
re-created by SpatializerHelper.init().

Bug: 224701940
Test: enable spatial audio and kill native audio server
Change-Id: Ib409e35f6aee9c149d6a998878f53209d74c54ad