Leaky internal engine in mobile

While working on #41149 , I discovered that `InternalEngine*` is saved as a raw pointer. This creates a leak which is manually suppressed with `absl::LeakIgnore`. I have tried using `unique_ptr` first, but that causes strange issues:
* mobile coverage fails with a mystery heap-use-after-free;
* Swift tests flake.

This should probably be addressed at some point with more robust lifetime.