Make ListenerMultiplexer more flexible

Previously ListenerMultiplexer attempted to be perscriptive about when
individual listener invocations should be allowed, but this is not
proving flexible enough. So, we allow registrations to invoke listeners
at any time without being perscriptive about it instead.

Test: manual + presubmit
Change-Id: I6ff8c6bd4f3e3af15d6ebc4b484b971df5a8c0e2