st-hal: Cache current state for merge sound model usecases

In a recent fix, handle_load_sm and handle_unload_sm transition
the session to loaded state if the session is stopped. The original
state needs to be cached before this happens in order to recover
back to the active state.

Change-Id: Ica978f565cb24da95f5fdff352a8652519ec9143
1 file changed