Add getData() member to SoundTriggerDetector.EventPayload.

If the HAL populates the recognition event with data that isn't trigger
audio, it's currently impossible for clients to actually read that
opaque data. By adding this getter, clients who understand how the
detection engine works can react to whatever is in the data blob.

Test: Modify SoundTriggerTestApp to verify that the data is accessible.
Change-Id: I8a9feccab98e2d15653dd55f28a43095f8ee1e44