ConntrackMonitor: enlarge the socket receive buffer size

Needed because FdEventsReader gets the error ENOBUFS while too many
connections are disconnected by losing network.

Manual test:
1. Enable IPv4-only mobile data as upstream.
2. Enable {USB, WIFI} tethering.
3. Create total 200+ connections from tethered clients.
4. Check the offload rules in dumpsys.
5. Disable mobile data.
6. Enable mobile data.
7. Create total 200+ connections from tethered clients.
8. Check the offload rules in dumpsys.

Test: atest NetworkStackTests

Change-Id: Id2d034e103c1ca3bd3a5935bb72c7d5520afc8b0