Remove system_current workaround on NetworkStack

Now that system_30 APKs can be built, use system_30 instead of
system_current when building NetworkStack.

Bug: 143586229
Test: m
Change-Id: If5246c9b8b1534df359cf3f1ec921f8de110e742
diff --git a/Android.bp b/Android.bp
index efe8ff1..ccdaf7a 100644
--- a/Android.bp
+++ b/Android.bp
@@ -48,8 +48,7 @@
 
 java_defaults {
     name: "NetworkStackReleaseApiLevel",
-    // TODO: use system_30 once a prebuilt SDK is available for R
-    sdk_version: "system_current",
+    sdk_version: "system_30",
     min_sdk_version: "29",
     target_sdk_version: "30",
 }