Implement the soundtrigger_middlewware service

This service is intended to replace:
frameworks/av/include/soundtrigger/ISoundTriggerHwService.h

This change only adds the replacement service, follow up
changes migrate the clients to use the new service and remove
the old one. The new service is feature-equivalent to the new
one, but offers the following advantages:
- AIDL interface (as opposed to hand-written parceling code).
- Pure Java implementation all the way to the HAL.
- Better documentation.
- Rigorous error handling.
- Unit tests.
- Reduced code complexity (less layers, better separation of
  concerns).
- Permission-based security model (as opposed to some baked-in
  assumptions about process affinity).

Change-Id: I79f4eff105d3e6245990be068b933d4d48c35a0d
Bug: 142070343