This CL robustifies the echo removal in AEC3 during the initial parts
of a call in two ways:
-By extending the period until which a headset is deemed to be used.
-By increasing the assumed echo path gain for unknown echo paths at
higher frequencies.
BUG=webrtc:7971
Review-Url: https://codereview.webrtc.org/2974883002
Cr-Commit-Position: refs/heads/master@{#18967}