Fix MultinetworkApiTest fail problem in instant mode

CtsNetUtils#storePrivateDnsSetting might require WRITE_SECURE_SETTINGS
permission to run.
Therefore, move it from setUp to only belong to the required test case.

Bug: 158538281
Test: run cts --include-filter "arm64-v8a CtsNetTestCases[instant]
android.net.cts.MultinetworkApiTest"

Change-Id: I8ecadd134824a4023a3c6e173d52ba088d46b103

tests/cts/net/src/android/net/cts/MultinetworkApiTest.java

@@ -68,7 +68,6 @@ public class MultinetworkApiTest extends AndroidTestCase {
         mCM = (ConnectivityManager) getContext().getSystemService(Context.CONNECTIVITY_SERVICE);
         mCR = getContext().getContentResolver();
         mCtsNetUtils = new CtsNetUtils(getContext());
-        mCtsNetUtils.storePrivateDnsSetting();
     }
 
     @Override
@@ -223,6 +222,7 @@ public class MultinetworkApiTest extends AndroidTestCase {
 
     @AppModeFull(reason = "WRITE_SECURE_SETTINGS permission can't be granted to instant apps")
     public void testResNApiNXDomainPrivateDns() throws InterruptedException {
+        mCtsNetUtils.storePrivateDnsSetting();
         // Enable private DNS strict mode and set server to dns.google before doing NxDomain test.
         // b/144521720
         try {