Remove Allocator-based overload

There are no known clients of registerNativeAllocation() that pass an
Allocator. We'd like to remove the functionality from
NativeAllocationRegistry.

Add comment about missing reachabilityFence() call.

Bug: 120823683
Test: TreeHugger
Change-Id: I2b6ec011e23e3d94a469d739db01e73421a36c1d