Host allocation follow-on work

Nobody has claimed this yet.

Noting some additional work items to pursue after https://github.com/llnl/axom/pull/1964 is merged (assuming it does get merged). Rather than address these items in that PR to keep it smaller and focused, these items can be addressed in a future PR. These additional changes affect Axom APIs and semantics when behavior is not explicitly specified.

- [ ] Change `axom::Array` to use `AXOM_DEFAULT_HOST_ALLOCATOR` choice rather than `MemorySpace::Dynamic` so that it's host allocation is consistent with the rest of Axom.
- [ ] Figure out how to avoid mixing `AXOM_MALLOC` enum value, which does not correspond to a valid Umpire allocator, with the rest of the enum values that do correspond to valid Umpire allocators.
- [ ] Change the name of `setDefaultAllocator` to `setUmpireDefaultAllocator` because the method only sets the default allocator in the Umpire ResourceManager?

Additionally, we will need a good way to test these changes in end user applications to make sure we don't break, or at least help them fix, their assumptions.

Start by reading PR #1964 and the definitions of axom::Array, the AXOM_MALLOC enum value, and setDefaultAllocator. Determine the intended API and semantic changes, then establish end-user application tests that verify allocator behavior and document what assumptions must remain valid or be updated.

Written by the indexing model from the issue text.