layers: Protect GetImage/BufferMemRqmts map access

layers: Protect GetImage/BufferMemRqmts map access

Lock 'global_lock' while looking for IMAGE_STATE/BUFFER_STATE in
GetBufferMemoryRequirements/GetImageMemoryRequirements. This is needed
because other threads could add/remove other VkImage handles to/from
the maps concurrently. The specs say the VkImage must be externally
synchronized, but this doesn't cover the internals of the layers about
VkImages in general. vkCreateImage, vkDestroyImage and
vkBindImageMemory/vkBindBufferMemory correctly lock 'global_lock'.

Change-Id: I93cb00c9a7f99efc927da52e42103ab72516397e