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
Merged-In: I3e3cbc5340a6e9f07359786cc931875a55e1f048
Change-Id: I3e3cbc5340a6e9f07359786cc931875a55e1f048
(cherry picked from commit d6bee3a2)