[Thread] define Thread hardware feature

Define Thread hardware feature FEATURE_THREADNETWORK = "android.hardware.threadnetwork"
so that apps/users can know whether the device has Thread support.

This flags is defined as public API but not system API because we
defined TRANSPORT_THREAD as public API and general apps can access
Thread network by adding callback for the Thread network. Adding
the FEATURE_THREADNETWORK as public API allows apps to be more
clear/confident of whether they can retrieve the Thread network
with TRANSPORT_THREAD.

FR: b/235016403

Bug: 284296494
Bug: 292948264
Change-Id: I3bce3458fa721a20d8937a0c70eec2dbc7c54bc6