Gracefully handle module death in SoundTriggerMiddlewareValidation

Previously, when the sound trigger module died (typically, when the
HAL process crashed), the SoundTriggerMiddlewareValidation module
would consider the client's attempt to detach() invalid, since it has
not unloaded the models. However, in this case, it is valid to detach
without unloading them.

Furthermore, dumpsys would not reflect the fact that the session is
dead. This is now fixed as well.

Fixes: 152523828
Bug: 152470531
Test: Manually killing audio server after sound models are loaded and
      inspection of the logs and dumpsys.
Change-Id: I96c6bcddb25a67ff440c0375b20036d57e5cd837