net: support for android without cgo

[crawshaw]

It is possible to build Android OS binaries (not apps) without cgo. Those apps will fail
because the Android OS does not define /etc/resolve.conf. Some more details on #8020 and
the linked CL.

[jhleath]

Please note that this issue is present when building with cgo for Android and looking up DNS records not supported by getaddrinfo() such as SRV.