Hold a reference to the scheduler for each vulkan device.

This attempts to work around Windows-specific teardown issues, possibly caused by dependency atexit() logic.

Bug: b/141380274
Change-Id: I5a34c18f8e3738f02221f1784419bc21a6430067
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36990
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>