frameworks/av: Fix format for 24 bit PCM upon fallback

Currently, stale format is in use when 24 bit pcm playback fallback
to non-offload playback. Because of this, WAV extractor pads data -
assuming pcm offload. By setting format as AUDIO_FORMAT_PCM_16_BIT
during fallback, proper data is read in extractor.

Change-Id: I775a73e1dd686a95f8b4bd49a26e6dc0b331f814
CRs-Fixed: 763633