Ignore checking TRANSPORT_USB when Android version is older than S

There is a new transport type - TRANSPORT_USB(8) in Android S,
so when the test tries to add this new transport type in older
Android version, it cannot pass the transport type validation and
make test fail.

Bug: 184158327
Test: run CtsNetTestCasesLatestSdk on Android R & Q
Change-Id: I38816173b04ea198d99f64f45e9271ac2641e4ac