Add back resources exported for external usage

These resources are not used by Settings, but is exported for external
usage by other apps.

Put them into res-export folder and add a README to prevent from
accidentally clean ups.

Bug: 234570979
Test: make Settings
Change-Id: Ia46e94f4ece6a40edf95b47188f45659650d443c

Android.bp

@@ -55,7 +55,11 @@ android_library {
     ],
 
     srcs: ["src/**/*.java", "src/**/*.kt"],
-    resource_dirs: ["res", "res-product"],
+    resource_dirs: [
+        "res",
+        "res-export", // for external usage
+        "res-product",
+    ],
     static_libs: [
         "androidx-constraintlayout_constraintlayout",
         "androidx.slice_slice-builders",