mm-audio: aac: modify aac encoding test app

modify aac test app to support pcm-mono to aac-stereo encoding
and pcm-stereo to aac-mono encoding

test app uses input pcm file sample rate for AAC encoding.
Modify test app to perform AAC encoding with sample rate
mentioned by user

Change-Id: Ic24860a21bb537dc39eca03d2a7153ceff16baba