Unifies the `TETHERING_LOCAL_NETWORK_AGENT` and
`WIFIP2PGO_LOCAL_NETWORK_AGENT` legacy feature flags into a
single mainline beta aconfig flag:
`tethering_and_p2p_go_local_network_agent`.
This change simplifies the logic for enabling the `NetworkAgent`
for both standard tethering (global scope) and Wi-Fi P2P Group
Owner mode (local scope). The agent is now enabled for both use
cases when the new flag is on.
The implementation in `IpServer` and associated tests have
been updated to use the new unified flag.
Flag: com.android.tethering.mainline.beta.tethering_and_p2p_go_local_agent
Bug: 420590461
Test: atest TetheringTests
Test: adb root
adb shell device_config put com_android_tethering com.android.tethering.mainline.beta.tethering_and_p2p_go_local_agent true
adb reboot
atest CtsTetheringTest --test-filter '.*Agent.*'
Test: atest ConnectivityCoverageTests:android.net.ip.IpServerTest --update-device
Change-Id: I55817132c57a04b4f7b7bac61b5741f45ba16182
dc420ccccb