Fix audio_decoder_unittests.isolate

In r6427 all .isolate files except
audio_decooder_unittests.isolate was updated to use the
<(DEPTH) variable instead of relative paths.
This started breaking the Android bots after committing
r7014.

BUG=3741
TBR=phoglund@webrtc.org,

Review URL: https://webrtc-codereview.appspot.com/23409004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@7017 4adac7df-926f-26a2-2b94-8c16560cd09d