Change ACM's CodecManager to hold one encoder instead of an array

With this change, the currently used encoder is held in a scoped_ptr.
iSAC is a special case, since the encoder instance is also a decoder
instance, so it may have to be available also if another send codec is
used. This is accomplished by having a separate scoped_ptr for iSAC.

Remove mirror ID from ACM codec database functions, and remove unused
functions from the database.

COAUTHOR=kwiberg@webrtc.org
BUG=4228
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/48729004

Cr-Commit-Position: refs/heads/master@{#8982}
5 files changed