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.

CYNGNOS-3303
Bug: 30907212
Change-Id: I6f02ec3fd1e6392b842b334e1cc4f9aa23916009
(cherry picked from commit 3b01b301)
(cherry picked from commit a41095d4)
(cherry picked from commit f2185115)