Enforce VPN control "permission" with an actual permission.

The current implementation uses a whitelist of package names. Use a
system|signature permission instead of rolling our own security and
add that permission to the existing set of whitelisted packages
(SystemUI and VpnDialogs).

In addition to being less of a security risk (using well-known methods
like Context.enforceCallingPermission rather than manually querying
PackageManager and checking UIDs for package names), this enables
other system-privileged apps to control VPN as needed per the below
bug.

Bug: 18327583
Change-Id: I38617965c40d62cf1ac28e3cb382c0877fb1275d
diff --git a/packages/SystemUI/AndroidManifest.xml b/packages/SystemUI/AndroidManifest.xml
index b3e60d1..5f024ac 100644
--- a/packages/SystemUI/AndroidManifest.xml
+++ b/packages/SystemUI/AndroidManifest.xml
@@ -55,6 +55,7 @@
     <uses-permission android:name="android.permission.MANAGE_NETWORK_POLICY" />
     <uses-permission android:name="android.permission.CONNECTIVITY_INTERNAL" />
     <uses-permission android:name="android.permission.READ_NETWORK_USAGE_HISTORY" />
+    <uses-permission android:name="android.permission.CONTROL_VPN" />
 
     <!-- Physical hardware -->
     <uses-permission android:name="android.permission.MANAGE_USB" />