Fix EthernetManager.addListener dependency in Tethering settings.

Ethernet service related files are going to be moved into Connectivity
module. EthernetManager.addListener(listener, exectuor) will be exposed
out as module-lib API. Replace the current API usage with the one to be
exposed.

android.os.HandlerExecutor is a hidden API which isn't visible to modules,
try to replace HanlderExecutor class with Handler.post itself although
Setttings can still access this API.

Bug: 210586283
Test: m
Change-Id: I618b43769c68897f4724fae8213181be1cc770c6