soundtrigger: Use "default" instance name for HAL service

"default" implies using "primary" legacy device module.
This doesn't change anything for binderized mode, because
the server was only registering itself as "sound_trigger.primary",
so it was the only possibility anyways.

Usage of "stub" (debugging) module is still possible
if audioflinger and corresponding services are built with
USE_LEGACY_LOCAL_AUDIO_HAL=true (added makefile check to ensure
that).

Test: "Ok Google" from screen off state on sailfish
Bug: 36570720
Change-Id: Ibef7fb312ce593d71255cfac71ba4e8f1dd28666