net: always use cgo for DNS on Android

Android has (had?) its own local DNS resolver daemon, also my fault:

https://android.googlesource.com/platform/system/netd/+/007e987fee7e815e0c4bc820f434a632b7a69a9d

And you access that via libc, not DNS.

Fixes #10714

Change-Id: Iaff752872ce19bb5c7771ab048fd50e3f72cb73c
Reviewed-on: https://go-review.googlesource.com/9793
Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/conf.go

@@ -85,7 +85,7 @@ func initConfVal() {
 
 // hostLookupOrder determines which strategy to use to resolve hostname.
 func (c *conf) hostLookupOrder(hostname string) hostLookupOrder {
-	if c.forceCgoLookupHost || c.resolv.unknownOpt {
+	if c.forceCgoLookupHost || c.resolv.unknownOpt || c.goos == "android" {
 		return hostLookupCgo
 	}
 	if byteIndex(hostname, '\\') != -1 || byteIndex(hostname, '%') != -1 {

src/net/conf_test.go

@@ -272,6 +272,18 @@ func TestConfHostLookupOrder(t *testing.T) {
 			},
 			hostTests: []nssHostTest{{"google.com", hostLookupCgo}},
 		},
+		// Android should always use cgo.
+		{
+			name: "android",
+			c: &conf{
+				goos:   "android",
+				nss:    nssStr(""),
+				resolv: defaultResolvConf,
+			},
+			hostTests: []nssHostTest{
+				{"x.com", hostLookupCgo},
+			},
+		},
 	}
 	for _, tt := range tests {
 		for _, ht := range tt.hostTests {