Add flag for read_all_external_storage_files

The flag is used in the long term solution of download files B&R
project(go/dd-br-download-files). It will be used in
- AppOpsManager which creates new appop
- os/Environment which creates new public api to read external files

The flag is added in the backup repo due to it's a B&R project and
teamfood will be launched inside the backup team.

Bug: 376598575
Test: FLAG_ONLY
FLAG: com.android.server.backup.enable_read_all_external_storage_files
Change-Id: I3c4e122fe6499a612b3986011bd021eb64c319af
diff --git a/services/backup/flags.aconfig b/services/backup/flags.aconfig
index fcb7934..28d3cae 100644
--- a/services/backup/flags.aconfig
+++ b/services/backup/flags.aconfig
@@ -69,3 +69,11 @@
     bug: "376661510"
     is_fixed_read_only: true
 }
+
+flag {
+    name: "enable_read_all_external_storage_files"
+    is_exported: true
+    namespace: "onboarding"
+    description: "Enables read all external storage files"
+    bug: "376598575"
+}
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