[network] add NetworkSpecifier#redact(long) and

NetworkSpecifier#getApplicableRedactions()

This is useful for custom NetworkSpecifier implementation to specify
what permissions are required to use it with the ConnectivityManager
APIs. See ag/33487708 for an example.

This will be called in ConnectivityService before accepting a network
request.

API-Coverage-Bug: 427649735
Bug: 425613703
Flag: com.android.net.thread.platform.flags.thread_mobile_enabled

Change-Id: I8ecd494e89a24c1a67eee28fd76557858fe64802