Set sdk_version 28 in aidl_interface

default value of sdk_version in aidl_interface is changed.("28" -> "system_current")
so sdk_version of this module have to be "28" because it is included in framework.jar.

Test: m -j
Bug: 126469673
Change-Id: I515b1b236a7212198c419432b7c9fc70c623c63a
diff --git a/Android.bp b/Android.bp
index 8ffb11d..a682759 100644
--- a/Android.bp
+++ b/Android.bp
@@ -862,6 +862,11 @@
         "core/java/android/net/ipmemorystore/**/*.aidl",
     ],
     api_dir: "aidl/networkstack",
+    backend: {
+        java: {
+            sdk_version: "28",
+        },
+    },
 }
 
 filegroup {