Fix EBADF in TTS synthesizeToFile method.

Recent TTS change altered how the TextToSpeech.synthesizeToFile method
operates. Previously, synthesis service was responsible for creating
output file. Now, client API creates a file and then sends opened file
descriptor using ParcelFileDescriptor.

On service side, I forgot to keep a reference to a ParcelFileDescriptor object.
When GC was removing it, it was closing underlying file descriptor, resulting
in a EBADF error for all following writes to the output file.

This change makes use of a ParcelFileDescriptor.AutoCloseOutputStream to keep a
reference to the ParcelFileDescriptor. It will be referenced until we are done
with writing.

Change-Id: I8327af0eaeabaebfbbd8816d959783e89086a7c5