libstagefright: Fixed for noise with AAC stereo recording

In AAC recoding, channel count is not getting considered
while copying pcm samples to AAC encoder input frame
buffer. This is causing encoder input frame buffer
corruption and which is finally resulting to noise on
recording clip.

Fix this by taking channel count into account while
copying pcm samples to input frame buffer.

Change-Id: Ic2287c8b32ef265ea19ba39856724ae965fc95ba
CRs-Fixed: 274020