[Android] Make vulkan backend as a .so library

[kirklandsign]

🚀 The feature, motivation and pitch

So that it can be loaded during runtime. Not necessarily compiled into libexecutorch_jni.so. This gives more flexibility.

Alternatives

No response

Additional context

No response

RFC (Optional)

No response

cc @SS-JIA @manuelcandales @cbilgin

[kirklandsign]

When I make vulkan_backend static_lib, got
U executorch::runtime::register_backend(executorch::runtime::Backend const&)

That's good. It depends on ET runtime register_backend symbol.

However, When I make vulkan_backend shared_lib, got

00000000003a20ac T executorch::runtime::register_backend(executorch::runtime::Backend const&)

Duplicated symbol!

[kirklandsign]

I probably have some proof of concept, but refactor required. https://github.com/pytorch/executorch/compare/experiment-so?expand=1