Restrict GnirehtetControlService to privilegied

We want to avoid the VPN to be started or stopped by anyone, so it must
require a permission.

Since intercepting the whole network traffic is critical, we want a
system permission.

In addition, we want to call it from shell, so we need an existing
system permission granted to shell:
<https://github.com/android/platform_frameworks_base/blob/master/packages/Shell/AndroidManifest.xml>

From the small permissions set matching these conditions, I chose
android.permission.WRITE_SECURE_SETTINGS.

Note: The following network-related system permissions granted to shell
may sound more appropriate:
 - android.permission.MANAGE_NETWORK_POLICY
 - android.permission.CONNECTIVITY_INTERNAL

Unfortunately, they were added only recently, so they are not granted to
shell for older versions:
<aosp-mirror/platform_frameworks_base@6a05eee>

app/src/main/AndroidManifest.xml

@@ -13,7 +13,8 @@
         android:theme="@style/AppTheme">
         <service
             android:name="com.genymobile.gnirehtet.GnirehtetControlService"
-            android:exported="true">
+            android:exported="true"
+            android:permission="android.permission.WRITE_SECURE_SETTINGS">
             <intent-filter>
                 <action android:name="com.genymobile.gnirehtet.START" />
                 <action android:name="com.genymobile.gnirehtet.STOP" />