LooperExecutor is only truly needed in WebSocketChannelClient because of
WebSocketClient from autobanh requiring thread to have a looper. So
LooperExecutor was left there but replaced everywhere else with built-in
singleThreadExecutor/singleThreadScheduledExecutor.
Motivation behind this change is that built-in class behaves better
under testing environment and doesn't require hacky
RobolectricLooperExecutor.
Review-Url: https://codereview.webrtc.org/1992213002
Cr-Commit-Position: refs/heads/master@{#12823}
7cf11476b9