DnsResolver: Make DnsException ctor public

Making the DnsException constructor public is useful for apps
using DnsResolver to write their tests, and for internal packages
that may want to implement the DnsResolver.Callback interface.

Test: for regression; CTS tests in b/208479811
Bug: 208464882
Change-Id: I14641688f53721c96e6df9596a7506912ba3aec0

framework/src/android/net/DnsResolver.java

@@ -164,7 +164,7 @@ public final class DnsResolver {
         */
         @DnsError public final int code;
 
-        DnsException(@DnsError int code, @Nullable Throwable cause) {
+        public DnsException(@DnsError int code, @Nullable Throwable cause) {
             super(cause);
             this.code = code;
         }