Make apex availability of net-utils-framework-common explicit

This java library is available to the following
1. NetworkStack, a platform app
2. TetheringNext, an app in com.android.tethering

The availability to (2) was done implicitly using a baseline map in
build/soong/apex/apex.go. Make this explicit in Android.bp

Bug: 281077552
Test: m nothing
Change-Id: I5738ef1fc153e7ef9ccdde512e61310b0fd1ddbb

staticlibs/Android.bp

@@ -295,6 +295,10 @@ java_library {
             "-Xep:NullablePrimitive:ERROR",
         ],
     },
+    apex_available: [
+        "//apex_available:platform",
+        "com.android.tethering",
+    ],
 }
 
 java_library {