Feature/exit node Android (#1916)

Support exit node on Android.
With the protect socket function, we mark every connection that should be used out of VPN.

util/net/protectsocket_android.go

@@ -0,0 +1,14 @@
+package net
+
+import "sync"
+
+var (
+	androidProtectSocketLock sync.Mutex
+	androidProtectSocket     func(fd int32) bool
+)
+
+func SetAndroidProtectSocketFn(f func(fd int32) bool) {
+	androidProtectSocketLock.Lock()
+	androidProtectSocket = f
+	androidProtectSocketLock.Unlock()
+}