Define PRIVATE_DNS_MODE_OPPORTUNISTIC locally

The type of ConnectivityManager#PRIVATE_DNS_MODE_OPPORTUNISTIC
has changed from String to int, but the String definition is still
needed to update to Settings.Global.PRIVATE_DNS_MODE, so the
simplest way is to define one locally.

Bug: 185311744
Test: atest CtsNetTestCases
Change-Id: Iafcd861714d8aca44cede658ed630f9d5afd5e59
Merged-In: Iafcd861714d8aca44cede658ed630f9d5afd5e59
    (Cherry-picked from ag/14232792)
diff --git a/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java b/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
index 88172d7..d5a26c4 100644
--- a/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
+++ b/tests/cts/net/util/java/android/net/cts/util/CtsNetUtils.java
@@ -18,7 +18,6 @@
 
 import static android.Manifest.permission.ACCESS_WIFI_STATE;
 import static android.Manifest.permission.NETWORK_SETTINGS;
-import static android.net.ConnectivityManager.PRIVATE_DNS_MODE_OPPORTUNISTIC;
 import static android.net.NetworkCapabilities.NET_CAPABILITY_INTERNET;
 import static android.net.NetworkCapabilities.TRANSPORT_CELLULAR;
 import static android.net.NetworkCapabilities.TRANSPORT_TEST;
@@ -87,6 +86,7 @@
 
     private static final int PRIVATE_DNS_SETTING_TIMEOUT_MS = 10_000;
     private static final int CONNECTIVITY_CHANGE_TIMEOUT_SECS = 30;
+    private static final String PRIVATE_DNS_MODE_OPPORTUNISTIC = "opportunistic";
     public static final int HTTP_PORT = 80;
     public static final String TEST_HOST = "connectivitycheck.gstatic.com";
     public static final String HTTP_REQUEST =