[Impeller] Start setup on Vulkan context earlier. about flutter HOT 3 OPEN

@jiahaog did some testing and noticed 1.7s dedicated to this on startup time and was wondering if we could move this earlier:

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In lieu of starting the pipeline creation process earlier, we could unlatch its completion with the UI thread, instead synchronizing at the point where someone uses a pipeline. That may incur a small overhead every time the pipeline is used but have much faster startup time.

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It looks like creating the PipelineVK's (PipelineVK::Create) is already decoupled in the sense that PipelineLibraryVK::GetPipeline already returns a future. However ContentContext::ContentContext will launch all of those async operations then immediate synchronize on a subset of them because of ContentContext::InitializeCommonlyUsedShadersIfNeeded.

If we want to keep ContentContext::InitializeCommonlyUsedShadersIfNeeded, one optimization would be to make sure those commonly used shaders were the one that are at the top of the worker threads queue. ContentContext::InitializeCommonlyUsedShadersIfNeeded may be a deoptimization itself.

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