[roll] Roll fuchsia [wlan] Replace wlan_common SendPtr with std AtomicPtr

This change replaces all uses of the bespoke `SendPtr` type with the
standard library `AtomicPtr` type. The `AtomicPtr` type is a thin
wrapper around a raw pointer that implements `Send` and `Sync` and
provides atomic access to the pointer itself. No uses of `SendPtr`
require replacing the wrapped pointer once wrapped, and all uses
rely on an owned value or exclusive reference. Thus, wlan-mlme only
uses the `new`, `into_inner`, and `get_mut` methods in
`AtomicPtr`. All wrapped pointers are thread-safe `*mut c_void`
pointers, so Rust code cannot meaningfully write/read the pointee and
wlansoftmac is carefully designed to use the pointers from different
threads.

I originally created the `SendPtr` type to step around using
`AtomicPtr` since, at the time, it was only necessary to make the
wrapped pointers implement `Send`, and the atomic load and store of
the pointer itself was not required. However, the migration of
`DeviceOps` to use async functions requires those pointers to
implement `Sync` in some cases as well. Since `AtomicPtr` implements
both `Sync` and `Send` and still requires an unsafe dereference to
access the pointee, the tradeoff of maintaining the separate `SendPtr`
type is no longer worthwhile.

Test: Successful iface creation, scan, connect, disconnect with iwlwifi on NUC
Multiply: fuchsia-pkg://fuchsia.com/wlan-mlme-tests
Multiply: fuchsia-pkg://fuchsia.com/wlansoftmac-rust-tests
Multiply: fuchsia-pkg://fuchsia.com/wlan-hw-sim-contemporary-privacy-tests
Multiply: fuchsia-pkg://fuchsia.com/wlan-hw-sim-legacy-privacy-tests
Multiply: fuchsia-pkg://fuchsia.com/wlan-hw-sim-rate-selection-tests
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1034093
Original-Revision: 928dadd49b3c91d4331063aa8f54a93759ef6084
GitOrigin-RevId: 5708da60ce1ebb97642d2c1cee899b9d149588e9
Change-Id: Ifcd54c1053fcbf9251499340f11979e982eb983e