ethernet: rename getInterfaces to getClientModeInterfaces

getInterfaces is a wrapper around EthernetTracker#getAvailableInterfaces
which only returns interfaces currently in client mode. Having this
function named getInterfaces() is confusing.

Test: TH
Bug: 269004812
Change-Id: Iaaaab5e4bed840e7ec101df87575ca1a14eac3a3