change: android does not have systemd

Android is counted as being linux too.

At least according to wikipedia android does not have systemd:
https://en.wikipedia.org/wiki/Systemd

dnscrypt-proxy/service_android.go

@@ -0,0 +1,10 @@
+// +build android
+
+package main
+
+func ServiceManagerStartNotify() error {
+	return nil
+}
+
+func ServiceManagerReadyNotify() {
+}

dnscrypt-proxy/service_linux.go

@@ -1,3 +1,5 @@
+// +build !android
+
 package main
 
 import "github.com/coreos/go-systemd/daemon"

dnscrypt-proxy/systemd_linux.go

@@ -1,3 +1,5 @@
+// +build !andoid
+
 package main
 
 import (