Use FatVector instead of SkAutoMalloc in VulkanManager.

Skia is looking to make SkAutoMalloc private and will be moving it.
Using FatVector instead will make this code both less dependent on Skia
internals and more performant.

Test: refactoring CL. Existing unit tests still pass.

Change-Id: If9de10059775b75e1ab89078eacede2e20e91299