soundtrigger: initialize mHwDevice member in constructor

audioserver crashes when sound_trigger legacy HAL does not exist on the device,
as mHwDevice is not initialized to NULL.

Bug: 32022304
Change-Id: I47642927e91c03817508d462cfcc58370e89b184
Signed-off-by: Thierry Strudel <tstrudel@google.com>