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
diff --git a/framework/src/android/net/TestNetworkManager.java b/framework/src/android/net/TestNetworkManager.java
index 4e89414..a174a7b 100644
--- a/framework/src/android/net/TestNetworkManager.java
+++ b/framework/src/android/net/TestNetworkManager.java
@@ -41,7 +41,6 @@
 
     /**
      * Prefix for tap interfaces created by this class.
-     * @hide
      */
     public static final String TEST_TAP_PREFIX = "testtap";