[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/AndroidTest.xml b/tests/unit/AndroidTest.xml
index 939ae49..2d32e55 100644
--- a/tests/unit/AndroidTest.xml
+++ b/tests/unit/AndroidTest.xml
@@ -15,7 +15,8 @@
 -->
 <configuration description="Runs Frameworks Networking Tests.">
     <target_preparer class="com.android.tradefed.targetprep.TestAppInstallSetup">
-        <option name="test-file-name" value="FrameworksNetTests.apk" />
+      <option name="test-file-name" value="FrameworksNetTests.apk" />
+      <option name="install-arg" value="-t" />
     </target_preparer>
 
     <option name="test-suite-tag" value="apct" />