Add package filter for system_server stubs

Should prevent accidental addition of APIs outside com.android,
which is where we want these APIs to be.

Also fix a bug in the permission filegroup where it wasn't
specifying the subpath of its code.

Bug: 149293194
Test: m update-api
Change-Id: I144ce6dbfe524106fc7de87318f66390c31b45af
diff --git a/apex/Android.bp b/apex/Android.bp
index e8f6e6b..adf4185 100644
--- a/apex/Android.bp
+++ b/apex/Android.bp
@@ -137,6 +137,7 @@
     name: "service-module-stubs-srcs-defaults",
     args: mainline_service_stubs_args,
     installable: false,
+    filter_packages: ["com.android."],
     check_api: {
         current: {
             api_file: "api/current.txt",