[Impeller] vulkan: there is no flag to enable the vulkan backend #128286
Description
see:
- https://github.com/flutter/engine/blob/220ece4d9faac1a48a3fa28ada315303f3fdec10/shell/platform/android/platform_view_android.cc#L54
- https://github.com/flutter/engine/blob/220ece4d9faac1a48a3fa28ada315303f3fdec10/shell/platform/android/platform_view_android.cc#L81