Android app should lookup IPv4 address of gateway
If the strongSwan Android app is configured with the FQDN of the gateway, and this FQDN has both A and AAAA DNS RR, and the Android device has IPv6 access then app will fail to set up the VPN.
The app will select the gateway's IPv6 address and succeeds in sending IKE messages to it, but it can't handle the responses it receives because HAVE_IN6_PKTINFO is not enabled. But enabling it wouldn't allow it to use IPv6 to a gateway running strongSwan on linux anyways since linux unfortunately doesn't support ESP-UDP over IPv6.
The app should try the gateway's IPv4 address, or at least have a settings for this.