audio: add support for FLAC and AMRWB+ decoding

Add support for QTI FLAC decoding by setting up
the component only if it is enabled and is not an
encoder.

Add audio group in mediacodec.rc so that AMRWB+
and other audio nodes can be accessed by it.

Change-Id: Ia35b0b9218fdf2584348a2a95bf0401c5065dedc