Change callback handling from Arc to Box

There is only ever one ref count for this callback object, and it's
always completely dropped when the Accessor or AccessorProvider objects
are dropped, so Box is a better type.

Test: atest rustBinderTest # has the *callback_destruction tests
Bug: 358427181
Change-Id: Ifc4b17a589b27f232c20c7e8a82d2b23a717fda8