Support LocalNetworkAgent for Tethering

The feature will be gradually enabled via flag rollout.

Sample ip rule output:
0:	from all lookup local
10000:	from all fwmark 0xc0000/0xd0000 lookup legacy_system
11000:	from all iif lo oif dummy0 uidrange 0-0 lookup dummy0
11000:	from all iif lo oif rmnet1 uidrange 0-0 lookup rmnet1
11000:	from all iif lo oif wlan0 uidrange 0-0 lookup wlan0
16000:	from all fwmark 0x10063/0x1ffff iif lo lookup local_network
16000:	from all fwmark 0x10064/0x1ffff iif lo lookup rmnet1
16000:	from all fwmark 0x10066/0x1ffff iif lo lookup wlan0
16000:	from all fwmark 0x10063/0x1ffff iif lo lookup wlan0
17000:	from all iif lo oif dummy0 lookup dummy0
17000:	from all iif lo oif rmnet1 lookup rmnet1
17000:	from all iif lo oif wlan0 lookup wlan0
18000:	from all fwmark 0x0/0x10000 lookup legacy_system
19000:	from all fwmark 0x0/0x10000 lookup legacy_network
20000:	from all fwmark 0x0/0x10000 lookup local_network
20000:	from all fwmark 0x0/0x10000 lookup wlan0
21000:	from all iif wlan0 lookup rmnet1
23000:	from all fwmark 0x64/0x1ffff iif lo lookup rmnet1
23000:	from all fwmark 0x66/0x1ffff iif lo lookup wlan0
31000:	from all fwmark 0x0/0xffff iif lo lookup rmnet1
32000:	from all unreachable

Test: adb shell device_config put tethering tethering_local_network_agent 1
Test: atest TetheringTests:android.net.ip.IpServerTest TetheringManagerTest
Bug: 349487600
Change-Id: Iaa88dde645da917149fec0d44fff796c551bcc7e
5 files changed