Set extensions_dir on prebuilt_apis

This means the extension versions will be taken into account when
figuring out what the latest finalized txt is for each module.

Bug: 220086085
Test: (mv extensions/1 extensions/2 && m nothing), inspect paths to
      latest api modules.
Change-Id: Id84cb58f3754c06a6d55b546688a2d933973ddb8
diff --git a/Android.bp b/Android.bp
index 91399e9..ef39487 100644
--- a/Android.bp
+++ b/Android.bp
@@ -134,6 +134,7 @@
         "32",
         "current",
     ],
+    extensions_dir: "extensions",
     next_api_dir: "33",
     imports_sdk_version: "none",
     imports_compile_dex: true,