Add a mutex to VulkanManager::initialize.

The initialize call can be made from both an GrallocUploadThread
and the RenderThread. So we need a mutex to make sure both
threads don't try to initialize in parallel.

Bug: 187218890
Test: manual running of device, guessing this could fix bug.
Change-Id: I203afd91fad5eacc131c839dff1a1065864b76f8