[SettingsProvider] fix connectivity test

TestOnly flag allows the test to read restricted @Readable setting
ALWAYS_ON_VPN_LOCKDOWN_WHITELIST.

BUG: 177239688
Test: atest com.android.server.ConnectivityServiceTest
Change-Id: If41708ad31de9cec3a102b4de4d237e96a448ef1
diff --git a/tests/unit/AndroidManifest.xml b/tests/unit/AndroidManifest.xml
index 887f171..54e1cd0 100644
--- a/tests/unit/AndroidManifest.xml
+++ b/tests/unit/AndroidManifest.xml
@@ -50,7 +50,7 @@
     <uses-permission android:name="android.permission.NETWORK_STATS_PROVIDER" />
     <uses-permission android:name="android.permission.CONTROL_OEM_PAID_NETWORK_PREFERENCE" />
 
-    <application>
+    <application android:testOnly="true">
         <uses-library android:name="android.test.runner" />
         <uses-library android:name="android.net.ipsec.ike" />
         <activity