Add a feature flag for USD

USD stands for Unsynchronized Service Discovery. It's a key feature
that allows devices to discover each other directly without needing a
time synchronization. It also enables service discovery and sending
messages using wifi action frames.

Bug: 340878198
Flag: android.net.wifi.flags.usd
Test: build successfuly
Change-Id: I1fd83a8543fb975968998fce98e6fe626951cd42