audio: Clear audio source if it is failed to be started

- fallbackToSWDecoder is not deleting mAudioSource, mOmxSource
  when there is an error while starting the source. Due to this,
  play_l calls stop() on mAudioSource even when mAudioSource is
  not started. This is leading to a crash in MPEG4Source
- Fix is to delete mAudioSource and mOmxSource if it is failed
  to be started

Change-Id: I6f555f5094b813a1e4ef3934c9e7a712ba41be02