Add a wrapper that uses anet on Android, but net on other platforms.
### Purpose
Fixes
https://forum.syncthing.net/t/workaround-for-android-local-discovery/20403/12
### Testing
Run two Syncthing instances with Global Discovery disabled. Pair them
with each other, don't hardcode their addresses, and verify they
connect.
### Purpose
As discussed on the forum:
https://forum.syncthing.net/t/reviving-nat-pmp-in-v2-x-on-android-14/24554
TL;DR
Android 14+ only lets java code get the gateway IPv4 address which is
used in SyncthingNative’s NAT-PMP feature.
So I’ve added the java code to the wrapper, got the router IP address
and feeded it to SyncthingNative by setting the env var
“FALLBACK_NET_GATEWAY_IPV4”.
This revives the NAT feature:
> [Z36WU] INFO: Detected 1 NAT service
### Testing
Local build and test via Android emulator (AVD 15).
