iSAC: Make separate AudioEncoder and AudioDecoder objects

The only shared state is now the bandwidth estimation info.
This reduces the amount and complexity of the locking
substantially.

Review URL: https://codereview.webrtc.org/1208993010

Cr-Commit-Position: refs/heads/master@{#9762}
diff --git a/webrtc/modules/audio_coding/audio_coding.gypi b/webrtc/modules/audio_coding/audio_coding.gypi
index 90a6f39..bc3c48d 100644
--- a/webrtc/modules/audio_coding/audio_coding.gypi
+++ b/webrtc/modules/audio_coding/audio_coding.gypi
@@ -15,6 +15,7 @@
     'codecs/g722/g722.gypi',
     'codecs/ilbc/ilbc.gypi',
     'codecs/isac/isac.gypi',
+    'codecs/isac/isac_common.gypi',
     'codecs/isac/isacfix.gypi',
     'codecs/pcm16b/pcm16b.gypi',
     'codecs/red/red.gypi',