Add auth protection for ssh key export

We use a builtin keyguard for ssh key export protection on Android.

This protection works only if some protection is set on the phone.

https://developer.android.com/reference/android/app/KeyguardManager#isDeviceSecure()

client/android/src/org/amnezia/vpn/AuthHelper.java

@@ -0,0 +1,24 @@
+package org.amnezia.vpn;
+
+import android.content.Context;
+import android.app.KeyguardManager;
+import android.content.Intent;
+import org.qtproject.qt5.android.bindings.QtActivity;
+
+
+import static android.content.Context.KEYGUARD_SERVICE;
+
+public class AuthHelper extends QtActivity {
+
+   static final String TAG = "AuthHelper";
+  
+   public static Intent getAuthIntent(Context context) {
+   	KeyguardManager mKeyguardManager = (KeyguardManager)context.getSystemService(KEYGUARD_SERVICE);
+   	if (mKeyguardManager.isDeviceSecure()) {
+                return mKeyguardManager.createConfirmDeviceCredentialIntent(null, null);
+        } else {
+        	return null;
+        }     
+   }
+   
+}