Explicitly close sound model file descriptors

This is a workaround for the fact that currently, relying on the GC to
release those resources is unreliable and on the other hand having an
explicit close() that isn't tightly coupled with the internal details
of the parcelable is not possible (filed a bug).

Test: Manual verification of soundtrigger operation.
Test: atest com.android.server.soundtrigger_middleware.SoundHw2CompatTest
Fixes: 219411458
Change-Id: I43bd43ca80eadf97f0254678b0e817fc6f8a06b2