Add some warnings and notes about framework's jarjar rules

These rules are explicitly recommended  to be depended on by other
libraries in the bootclasspath, that may depend on the framework's
copy of the hidl library.

Bug: 186804505
Test: presubmit
Change-Id: I09766423782d9bb9408f3d14169d7e5d8f1b5823

Android.bp

@@ -271,9 +271,12 @@ java_library {
     installable: false,
 }
 
+// NOTE: This filegroup is exposed for vendor libraries to depend on and is referenced in
+// documentation. Do not remove without consulting the treble/hidl teams.
 filegroup {
     name: "framework-jarjar-rules",
     srcs: ["framework-jarjar-rules.txt"],
+    visibility: ["//visibility:public"],
 }
 
 java_defaults {

framework-jarjar-rules.txt

@@ -1,3 +1,7 @@
+# This rule is meant to be reused across libraries in the bootclasspath that depend
+# on the HIDL libraries.
 rule android.hidl.** android.internal.hidl.@1
+
+# Framework-specific renames.
 rule android.net.wifi.WifiAnnotations* android.internal.wifi.WifiAnnotations@1
 rule com.android.server.vcn.util.** com.android.server.vcn.repackaged.util.@1