pw_bluetooth_sapphire: Fix UAF in LogicalLink::SignalError

Avoid a Use-After-Free (UAF) vulnerability by invoking the link error
callback asynchronously.

When all channels on a logical link are closed, SignalError invokes the
link error callback. In LE fixed-channel-only links, this can happen
synchronously, which in turn synchronously destroys the LogicalLink
and its channels. Returning from SignalError (or the channel's
SignalLinkError) would then execute on a destroyed object. By posting
the error callback to the dispatcher using HeapDispatcher, we ensure
that the LogicalLink is only destroyed after the call stack has
completely unwound.

Fixed: b/510065734
Fixed: b/535162699
Change-Id: I1463dd717982fa1b932a12aa6b0d1f34fc91e64a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/482629