Set sticky bit on encryptedstore

Also move this & the previous change behind a cfg, to preserve
existing behavior until we're ready. Wire the cfg up to the
new flag.

Bug: 296395087
Test: Run VM, examine directory
Test: Override flag locally, atest MicrodroidTests
Change-Id: If4ddf3384ee7324768ba4adb6e426e31032e5b32
diff --git a/encryptedstore/Android.bp b/encryptedstore/Android.bp
index 8ba5016..aa46c35 100644
--- a/encryptedstore/Android.bp
+++ b/encryptedstore/Android.bp
@@ -4,6 +4,7 @@
 
 rust_defaults {
     name: "encryptedstore.defaults",
+    defaults: ["avf_build_flags_rust"],
     srcs: ["src/main.rs"],
     edition: "2021",
     prefer_rlib: true,