Fix PrivateDnsPreferenceControllerTest test case failed

- getAvailibilityStatus_availableByDefault()

Fixes: 280048011
Test: locoal robotest
Change-Id: I490dc3348af241a76767885b52238d0b87880532

tests/robotests/src/com/android/settings/network/PrivateDnsPreferenceControllerTest.java

@@ -169,6 +169,7 @@ public class PrivateDnsPreferenceControllerTest {
 
     @Test
     public void getAvailibilityStatus_availableByDefault() {
+        doReturn(true).when(mUserManager).isAdminUser();
         assertThat(mController.getAvailabilityStatus()).isEqualTo(AVAILABLE);
     }