Adding a new method to the Expand class, which will answer the question
whether an ongoing expansion has been faded down to zero
amplitude (i.e., been muted). Also adding a test.
This new functionality will be used in CLs to follow.
BUG=webrtc:5608
NOTRY=True
Review-Url: https://codereview.webrtc.org/1967473004
Cr-Commit-Position: refs/heads/master@{#12672}