AudioEncoderDecoderIsac: Merge the two config structs

This patch merges the Config and ConfigAdaptive structs, so that iSAC
has just one config struct like the other codecs. Future CLs will make
use of this.

COAUTHOR=henrik.lundin@webrtc.org
BUG=4228
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/45979004

Cr-Commit-Position: refs/heads/master@{#9015}
4 files changed