Clean up prebuilt stub deps & add comment

The non-updatable stub depends on a prebuilt platform stub, so that it
can reference APIs defined in modules. Clean up the list of previously
needed prebuilts that are no longer necessary, and add a comment about
when it's suitable to add new prebuilts to this list.

Test: m
Change-Id: Idd5f57c774db4fc48abe4bab87c1b393079cae4d
diff --git a/api/StubLibraries.bp b/api/StubLibraries.bp
index d15b44f..e7dcbd0 100644
--- a/api/StubLibraries.bp
+++ b/api/StubLibraries.bp
@@ -348,13 +348,15 @@
     ],
     srcs: [":module-lib-api-stubs-docs-non-updatable"],
     libs: [
+        // We cannot depend on all-modules-module-lib-stubs, because the module-lib stubs
+        // depend on this stub. We resolve dependencies on APIs in modules by depending
+        // on a prebuilt of the whole platform (sdk_system_current_android).
+        // That prebuilt does not include module-lib APIs, so use the prebuilt module-lib
+        // stubs for modules that export module-lib stubs that the non-updatable part
+        // depends on.
         "sdk_module-lib_current_framework-tethering",
         "sdk_module-lib_current_framework-connectivity-t",
-        "sdk_public_current_framework-bluetooth",
-        // NOTE: The below can be removed once the prebuilt stub contains bluetooth.
         "sdk_system_current_android",
-        // NOTE: The below can be removed once the prebuilt stub contains IKE.
-        "sdk_system_current_android.net.ipsec.ike",
     ],
     dist: {
         dir: "apistubs/android/module-lib",