Support multiple clients attaching to a module

Currently service allows only a single client to be attached to a Module.
This limits only a single client can use sound trigger at a time.

Add changes to attach multiple clients to a given Module through ModuleClient
interface so that multiple clients can paralelly use sound trigger on a given
Module. ModuleClient class is introduced as a client interface to a Module.
Service provides a unique instance of ModuleClient to each client being attached
and adds this client to the module clients list.

Test: Manual with modified version of SoundTriggerTestApp that talks to
the module directly

Bug:32030191
Change-Id: I66912e862b8c4232a49d92c0464a9a8b876bdb26