Remove double check interface by NetworkInterface

NetworkInterface#getByName can not get the interface without IP addresses.
When setIncludeTestInterfaces(true) is called, the interface will be
placed in client mode, which will delete the link-local address. So calling
NetworkInterface#getByName to get test interface may not work before
switching to server mode.

Bug: 168215721
Test: atest CtsTethering
Change-Id: Ice8adcce1c4b2c86bd219c701b2afa9ba2339f24