Restrict audioformat to 8bit, 16bit or float PCM with an annotation and

a warning.

TextToSpeech.synthesizeToFile has only ever worked with 8bit and 16bit
PCM. In case of float PCM an invalid file would be generated and in case
of other encodings the TTS engine would crash.

Also add more annotations to SynthesisCallback.

Change-Id: I1c44a44509e9b53bb2e1b0f2044b1a5919875e1c