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: Ic1c7914aaf28ed9b86d6d4a5924c6c10f58f1f3c
diff --git a/apexd/Android.bp b/apexd/Android.bp
index 7dbdbcc..553f4c2 100644
--- a/apexd/Android.bp
+++ b/apexd/Android.bp
@@ -61,6 +61,9 @@
   ],
   local_include_dir: "aidl",
   backend: {
+      java: {
+          sdk_version: "28",
+      },
       ndk: {
           enabled: false,
       },