SoundTrigger: get service by value

Get strong pointer to service interface by value and not reference to
prevent race conditions where the service pointer can be cleared
by another thread while in use.

Bug: 30907212
Change-Id: I6f02ec3fd1e6392b842b334e1cc4f9aa23916009
(cherry picked from commit 3b01b301)
(cherry picked from commit 3a03fa24d21f97e84e796ac5ef14b3f434c0e8f1)