Expose TEST_TAP_PREFIX as system API

TestNetworkManager is a part of connectivity mainline module and
it is already a system API, but its constant - TEST_TAP_PREFIX
is hidden, so expose it for the callers inside framework.

Bug: 172183305
Test: make update-api
Change-Id: I7fc08be4a6ca6325d5b576b761afda17865cefaa

core/api/module-lib-current.txt

@@ -59,6 +59,7 @@ package android.net {
     method @NonNull public android.net.TestNetworkInterface createTunInterface(@NonNull java.util.Collection<android.net.LinkAddress>);
     method public void setupTestNetwork(@NonNull String, @NonNull android.os.IBinder);
     method public void teardownTestNetwork(@NonNull android.net.Network);
+    field public static final String TEST_TAP_PREFIX = "testtap";
   }
 
   public final class UnderlyingNetworkInfo implements android.os.Parcelable {

packages/Connectivity/framework/src/android/net/TestNetworkManager.java

@@ -41,7 +41,6 @@ public class TestNetworkManager {
 
     /**
      * Prefix for tap interfaces created by this class.
-     * @hide
      */
     public static final String TEST_TAP_PREFIX = "testtap";