Remove NativeAllocationRegistry.Allocator

It was a nice idea that never really got used. We don't want to
support it in the core platform API. Remove all traces.

Update NativeAllocationRegistryTest while we're at it, to
include some minimal testing of the new factory functions.

Bug: 120823683
Test: TreeHugger
Change-Id: Id9c6e1013315c2024c3ac0599dde99f34a7d61ee