Fix deletion of VkSemaphores in VulkanManager.

We were deleting the VkSemaphore objects too quickly when
importing/exporting the semaphores. Even though the semaphore payload
gets reset on these operations the VkSemaphore still needs to be
finished its use in Vulkan before being deleted.

Test: manual build and testing of vulkan apps and vulakn ImageConsumer
Bug: b/130643604
Change-Id: I7f03087e477d812c0174ede3a10f12dc1df72ee1