build: vendor pinned flutter_vless Android runtime

Replace build-time pub-cache mutation with a repository-owned pinned runtime so reviewed native source is the source compiled into the app.

local_packages/flutter_vless_android-1.1.5/UPSTREAM

@@ -0,0 +1,4 @@
+Upstream: https://github.com/XIIIFOX/flutter_vless
+Tag: v1.1.5
+Android package: flutter_vless_android 1.1.5
+ReVolt hardening is pre-applied to this vendored package.