[Impeller] Document and slightly refactor ResourceManagerVK & friends.

[matanlurey]

@chinmaygarde specifically, let me know if I violated any correctness you were looking for, or if you would prefer I don't make 1 or more of the changes I made here.

@gaaclarke would love your input on C++ style specifically, or anything that stands out in general.

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Summary

I plan to use the resource strategy in flutter/flutter#133198, but wanted to understand the code better first, so this is my contribution to make it a bit easier to understand (hopefully! push back if not!) and contribute tests.

1. Renamed Reset(ResourceType) to Swap() for consistency with the std smart pointers.
2. Moved non-trivial work out of the constructor into ::Create (style guide).
3. Added some FML_DCHECKs to private APIs to enforce correctness.
4. Made classes final and methods private where they were effectively that (style guide).
5. Added tests to make sure I understood the contracts.

[matanlurey]

@chinmaygarde QQ:

if (auto manager = resource_manager_.lock()) {
  manager->Reclaim(std::move(resource_));
}

... What if another thread calls Reclaim() without locking? Should that also lock?

[matanlurey]

I'm still working on the tests, they were more aspirational than functional :)

[gaaclarke]

LGTM. I'd just add a test that clears out a resource to capture coverage for most of the code.

[chinmaygarde]

LGTM pending Aarons comments. Thanks!

[matanlurey]

LGTM pending Aarons comments. Thanks!

@chinmaygarde Could you respond to #45474 (comment)?

[chinmaygarde]

What if another thread calls Reclaim() without locking? Should that also lock?

Weak pointers can't be used that way. You need to convert the weak to a shared pointer using a lock. So, if you can call reclaim, you need a shared pointer (using a lock or otherwise).

[gaaclarke]

It's not quite right yet. Sorry, it's best to make it flakeproof however unlikely the flake.

[gaaclarke]

lgtm!