…hread-local command pools (flutter#169548)

The Impeller Vulkan back end creates a thread-local map of contexts to
CommandPoolVK instances for each thread that uses Vulkan. This allows a
thread to obtain the CommandPoolVK that is currently in use for a given
context.

When a context is shut down, the Vulkan resources used by each thread's
local CommandPoolVK for that context must be freed. To do this,
CommandPoolVK maintains a global map of CommandPoolVK instances.

Prior to this PR Impeller was appending to the context's pool list in
the global map each time a new CommandPoolVK was created. In the
original implementation this worked because Impeller was only creating
one CommandPoolVK per thread for a given context.

However, CommandPoolVK later adopted a recycling scheme where each frame
creates a new CommandPoolVK instance that acquires a Vulkan command pool
from the CommandPoolRecyclerVK. So inserting every CommandPoolVK into
the global map will cause the global map to grow unbounded.

This PR changes the structure of the global map. The global map will now
associate each context with a map of thread IDs to the CommandPoolVK
that is currently placed in the thread's local storage. When a thread
calls CommandPoolRecyclerVK::Dispose to clear its thread-local
CommandPoolVK for a context, the corresponding entry is also removed
from the global map.

Fixes flutter#169208