Spatializer: enforce only one client registered

Make sure that only one client can successfully obtain an
ISpatializer control interface to the native Spatializer controller.
Do not reject the same callback being registered twice (NOOP).

Bug: 244427550
Test: manual spatial audio regression
Change-Id: I3e3cbc5340a6e9f07359786cc931875a55e1f048
2 files changed