Create a repo rule & module_extension for android sdk proxy

@android_external will be Bazel's direct dependency with dummy/falsy value. In android projects, define @android_external with each target pointing to android sdk content.

This is prerequisite to retire //external:android... in bazel/rules_android.

PiperOrigin-RevId: 611595652
Change-Id: I3f308b969b00d394b324c7cef789f79372a1583c
diff --git a/MODULE.bazel b/MODULE.bazel
index 83f6b34..eb2be76 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -362,3 +362,6 @@
 
 remote_android_extensions = use_extension("//tools/android:android_extensions.bzl", "remote_android_tools_extensions")
 use_repo(remote_android_extensions, "android_gmaven_r8", "android_tools")
+
+android_sdk_proxy_extensions = use_extension("//tools/android:android_extensions.bzl", "android_sdk_proxy_extensions")
+use_repo(android_sdk_proxy_extensions, "android_external")