Add SkipNativeNetworkCreation flag in NetworkAgentConfig

This flag allows ConnectivityService to skip native network creation,
deferring this responsibility to Tethering. This is necessary because:

- ConnectivityService requires the NetworkAgent to be in the CONNECTED
  state to configure routing and invokes NetworkCallback#onAvailable
  once ready.
- Tethering requires lower-layer interactions to be completed before
  notifying applications.

This approach ensures proper synchronization between ConnectivityService
and Tethering during network setup.

Long-term, all netd operations should be handled by ConnectivityService
when it supports early native network creation in the CONNECTING state.

As a mid-term solution, this flag enables Tethering to manage native
network creation within the IpServer.

Test: atest ConnectivityCoverageTests:android.net.connectivity.android.net.NetworkAgentConfigTest
Bug: 349487600
Change-Id: I84da9c74dc4d40c7cd36c6657f17a158affed689
2 files changed