Support null audio format in recognition event

The audio format doesn't always make sense in recognition events, for
example in a failure or abort event, so we allow it to be null.
However since the SoundTrigger.java API doesn't allow that, we inject
a default if it is not available.

Bug: 157496890
Test: Simulated a device that doesn't support concurrent capture,
      reproduced the bug, applied the fix, verified.
Change-Id: I1e4adf9f3ccdc0f62cb9ca5e8c07df3c40a9d2cd