API review: make exception class and Builder final

- InvalidPacketException exception class should be final
- NetworkCapabilities.Builder should be final

Bug: 152203926
Test: atest FrameworksNetTests
Change-Id: If9b799151aff6d41c9bcd8bb86c65a58e46bad73

core/java/android/net/InvalidPacketException.java

@@ -27,7 +27,7 @@ import java.lang.annotation.RetentionPolicy;
  * @hide
  */
 @SystemApi
-public class InvalidPacketException extends Exception {
+public final class InvalidPacketException extends Exception {
     private final int mError;
 
     // Must match SocketKeepalive#ERROR_INVALID_IP_ADDRESS.

core/java/android/net/NetworkCapabilities.java

@@ -2000,7 +2000,7 @@ public final class NetworkCapabilities implements Parcelable {
      */
     @SystemApi
     @TestApi
-    public static class Builder {
+    public static final class Builder {
         private final NetworkCapabilities mCaps;
 
         /**