EthernetTetheringTest: use EthernetManager to check iface before T

For cuttlefish before T, TetheredInterfaceRequester may be responded
to over 200ms. That causes tests to become flaky. Using ethernet
manager to check available tethering interface instead of repeated
increases TetheredInterfaceRequester timeout
AVAILABLE_TETHER_IFACE_REQUEST_TIMEOUT_MS.

Bug: 243314243
Test: atest EthernetTetheringTest

Change-Id: I9cfab19adf1a54be82040da225c89b6570a36d4c
1 file changed