wifi(vts): simplify HIDL calls

Presently, the core of the Wifi test logic is
obscured by the boilerplate required to create
a callback.

This CL provides some utilities to simplify
the creation of a HIDL result callback, and
migrates existing Wifi code to use the new
utilities.

Along the way: add a .clang-format file, so
that I don't misformat code with 2-space
indents (the Google default).

Bug: 34817351
Test: vts-tradefed run commandAndExit vts --module=HalWifiHidlTargetTest
Change-Id: Id2c728f96c3369c74adc8dfce7228b0a15a0781e