Fix race when handling callback in SoundTrigger Module

During the callback event processing loop in the SoundTrigger::Module,
it used to grab the ISoundTriggerClient from the ModuleClient that it
needed to send an event to. However, this could be removed/destroyed
while the callback thread is trying to snapshot it, since there was no
common locking between the model client and the module.

By now using the ModuleClient directly in the Module's callback, the
appropriate locks can be obtained to prevent segfaults.

Test: On device with GSA/Now Playing
Change-Id: I9e3d150aa77051caab1fc009c3fc1ae8eec61a72