This consolidates all relevant logic in a single place.
We no longer need to interact with ContextVk in the worker thread.

This switches the fixed pointer array size to a dynamically sized
vector. Some of the EGL and ANGLE tests would use a large number of
Contexts and we were consistently running out of available command
buffers which would cause a deadlock situation. We can trust other
parts of the code to throttle the application if it starts to get
too far ahead of the device and dispense with the hard coded limit
in the command buffer allocator itself. The resulting code is also
quite a bit simpler and doesn't need a condition variable.

Also fixes missing initialization in SecondaryCommandBuffer.

Bug: b/172704839
Change-Id: Icc3a3daf5d6b272db556c0e4c93fb793583966a5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2525143
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>