Disable stubs generation when generating module_lib .txt file

Avoids droidstubs doing unnecessary work to create stubs files that are
not actually used.

Bug: 146727827
Test: framework-sdkextensions-api-module_libs_api
Change-Id: Ic537fda99b564877709666025fb8df872144d827

StubLibraries.bp

@@ -193,6 +193,10 @@ droidstubs {
     api_tag_name: "MODULE_LIB",
     arg_files: ["core/res/AndroidManifest.xml"],
     args: metalava_framework_docs_args + module_libs,
+
+    // Do not generate stubs as they are not needed
+    generate_stubs: false,
+
     check_api: {
         current: {
             api_file: "api/module-lib-current.txt",

apex/Android.bp

@@ -122,6 +122,10 @@ stubs_defaults {
     installable: false,
     sdk_version: "module_current",
     filter_packages: framework_packages_to_document,
+
+    // Do not generate stubs as they are not needed
+    generate_stubs: false,
+
     check_api: {
         current: {
             api_file: "api/module-lib-current.txt",