Replace useTls with mode in PrivateDnsConfig

In PrivateDnsConfig class, `useTls` is replaced with `mode`.
This CL a corresponding change.

Bug: 240259333
Test: atest ConnectivityCoverageTests
Change-Id: I60c53e52f287fc7fa05543603a62632d934bb2e6
diff --git a/service/src/com/android/server/ConnectivityService.java b/service/src/com/android/server/ConnectivityService.java
index 8f29078..3252acb 100755
--- a/service/src/com/android/server/ConnectivityService.java
+++ b/service/src/com/android/server/ConnectivityService.java
@@ -4784,7 +4784,7 @@
         // If the Private DNS mode is opportunistic, reprogram the DNS servers
         // in order to restart a validation pass from within netd.
         final PrivateDnsConfig cfg = mDnsManager.getPrivateDnsConfig();
-        if (cfg.useTls && TextUtils.isEmpty(cfg.hostname)) {
+        if (cfg.inOpportunisticMode()) {
             updateDnses(nai.linkProperties, null, nai.network.getNetId());
         }
     }