wifi(interface): Rename interface to iface

The HIDL objects are called "interface", so rename the wifi interface to
iface to distinguish between them better.

This is especially needed in the follow up CL.

Bug: 31943042
Test: mmm -j32 hardware/interfaces/wifi/1.0/
Change-Id: I3bc34930f75159d26321c0e071af9084d0585508