[Thread] add permission THREAD_NETWORK_TESTING for guarding testing APIs or shell commands

For new Thread network APIs or shell command for only testing, a new
signature permission is deversed to protect them (so to distinguish them
from existing system APIs which are designed for privileged apps such as
GMS Core).

Bug: 329368792
Change-Id: I291989eb0e024614568044945726edc07e20b8be