Compile com.android.dex from AOSP Dalvik source

This class is not easily available on gMaven. Some other companies have their own uploads of this dep, but it's better to just build it from source here.

Uses AOSP commit 5a81[...], consistent with the legacy bazel/third_party version.

PiperOrigin-RevId: 758880284
Change-Id: I452026bda7656132596b064eeca0d0d5d3314c1f

bzlmod_extensions/BUILD

diff --git a/bzlmod_extensions/BUILD b/bzlmod_extensions/BUILD
index 858b3ce..14f924b 100644
--- a/bzlmod_extensions/BUILD
+++ b/bzlmod_extensions/BUILD
@@ -1,5 +1,6 @@
 exports_files(
     [
         "apksig.BUILD",
+        "com_android_dex.BUILD",
     ],
 )