Stop exposing constructor of TetheringConstants

Fully-static utility classes must not have constructor.

Bug: 149430003
Test: -build, flash, boot
      -atest TetheringTests

Change-Id: I18e1129ef8717211d667ccd3b4110169dd7905b4

Tethering/common/TetheringLib/src/android/net/TetheringConstants.java

@@ -33,6 +33,9 @@ import android.os.ResultReceiver;
  */
 @SystemApi(client = MODULE_LIBRARIES)
 public class TetheringConstants {
+    /** An explicit private class to avoid exposing constructor.*/
+    private TetheringConstants() { }
+
     /**
      * Extra used for communicating with the TetherService. Includes the type of tethering to
      * enable if any.